Am I the only one that thinks Ashfur wasn't evil?

No, Squirrelflight didn't use Ashfur to make Brambleclaw jealous! Not only was this confirmed by Vicky herself, but we literally have an entire book on the whole love triangle in which Squirrelflight has a PoV (Twilight). Just because Brambleclaw actually got jealous doesn't mean anything, never mind that it would be extremely OOC for her to be that petty. Squirrelflight even thinks that there could possibly be something with Ashfur, but she wanted to be absolutely sure of her feelings for Brambleclaw first before taking things further. It's not her fault things didn't work out the way Ashfur wanted them to.

Sorry for the rant, I'm not angry at you, just really tired of seeing this claim be made.

And as for Ashfur himself, no, it actually does make sense. Seriously, listen or watch to true crime podcasts or documentaries for a little while, and you'd probably be surprised how many good people just end up snapping one day because they couldn't get their dream job or the person they liked or whatever. Morality isn't black-and-white, it's gray. It's complex, it's a spectrum. And how many times are you going to come across tyrants—not just bigots, tyrants—like Brokenstar or Tigerstar? Not very likely. But Ashfurs? They're, unfortunately, everywhere, and that's exactly what makes him the scariest villain in the series to me.

Maybe Ashfur's turn could've been handled better, but even in the first arc, there really wasn't much going for him other than the fact he loved his family, which included Cloudtail for a time, and was quick to underestimate BloodClan just because they wore collars.

TPB was also originally going to be only one arc, but then it got popular enough to warrant more books, which meant Ashfur gaining more of a personality. And if you read the books in chronological order, there's no reason to think he may have developed a bit of an ego after avenging his parents at such a young age, hence his portrayal in GV. It was also already established that he wasn't very fond of outsiders and Graystripe had just been made deputy despite not having been in the Clan for very long, on top of the position having once belonged to his father Whitestorm, so if we look at it from his perspective, his resentment makes sense.

So what about TNP? Well, he mellowed out. That is, until he fell for Squirrelflight and became protective of her, which again, makes sense, considering he's already lost kin in the past and life in the wild is already hard as is. But then she chooses the son of his mother's murderer and slowly starts to snap, only managing to nearly kill her kits seasons later because of opportunity and tried to get revenge a second time because of opportunity.

Also, even if none of this was the case, nothing justifies him lashing out at Squirrelflight's kits—never mind trying to murder them—regardless of his past with her. They were completely innocent.

Not always. He was portrayed as likable, loyal, and brave enough in the first series, which itself was supposed to be self-contained.

The change was likely meant to be initiated around halfway through the second series, when the Erins decided that Brambleclaw and Squirrelflight needed a romantic rift. Can't say for sure around when they decided he was going from unlucky suitor to butthurt incel, but it was at least by Sunset. Birchpaw's message that led Firestar into the trap, Ashfur appearing on the scene to frame Brambleclaw, and Hawkfrost claiming Firestar's assassination attempt was an inside job makes it clear that it had to be him. And Lionpaw's apprenticeship makes it pretty clear that Ashfur is totally still asshurt behind the friendly, responsible facade.

Though it is worth noting that he was granted StarClan and that they outright tried to wave it off with "he loved too much". Part of me feels like they REALLY wanted a dramatic parentage reveal for the Three and a leadup to a catastrophic Season 1 finale for that arc, and Ashfur was very convenient for that setup. If I remember correctly, it has been stated in an official capacity that the fire scene was written almost before anything else in the Power of Three. So they set him up as a vengeful incel in Sunset, spring the surprise, get him murdered, and then sweep him into the dusting pan.

I think they eventually realized that nobody was accepting "he only loved too much", thought about it, and decided to take that elsewhere. I've said it before, but I think the Erins have a somewhat warped and sometimes unhealthy perception of love that manifest in their writing. It smacks of some of the worst shoujo manga tropes, but framed in Western literature about talking cats. So they genuinely thought making him a tragic villain made sense, and went back and took advantage of some untread ground to slip seem seeds hinting at him always being bad in his past while going on to create his future as the tyrannical Elliot overlord of ghosts.

Tl;Dr: definitely not when his character was conceived. They decided they'd make him a "tragic" villain starting around Sunset. And then they finally decided to drop the tragic part and embraced the idea of a deranged incel wreaking havoc from beyond the grave starting with the conception of TBC.

no he just loved too much

One of the things I was really impressed by in TBC and GV is that the editors did their research into Ashfur's almost nonexistent personality back then, and built off it so it was at least believable. They remembered he was very xenophobic and prejudiced, was immensely possessive of Squirrelflight, constantly manipulated, guilted, and argued with things he didn't like or to try and get his way. I think his downfall from an arrogant but still well-meaning warrior to an omnicidal incel works because he was such a minor character, but that's just me. 

From the start there always was something off with Ashfur. Just like Mapleshade there was a selfish, petty meanness to them. They held grievances and didn’t care who they hurt in order to get their way.

The shift to Ashfur being a darker character begins in TNP. Even before Squirrelflight returns to the clans, he's already become kind of a sneering character akin to Dustpelt and Mousefur, but even more pronounced. Notably of the group, he's the one who suggests Cloudtail left the clan for kittypet food in a rather callous fashion. Now this doesn't necessarily mean villain lead-up, but the seeds were present in the author's mind that he's sort of a judgy, haughty cat. This almost certainly is why he was made to be Squirrelflight's fake pairing, and by the time that he's helped Hawkfrost murder Firestar, he's transitioned into full-on villain status. However, I do believe it is true that at this time, he wasn't considered to be as evil as he is now, and the authors seemed to be operating under the idea that he was a sympathetic villain who only loved too much. That then marks the final transition to his current characterization, which doesn't really involve any change to Ashfur himself so much as it does the reaction of other characters to him, as they no longer justify him and instead condemn his actions.

In short, the author's perspectives appear to be

TPB Ashfur: Basically a nothing side-character, seems to fit well with Dustpelt
Early TNP Ashfur: Kind of a bigger jerk than the other clan cats, even more than his mentor
Late TNP and PoT Ashfur: Opportunistic villain, who will do terrible things when given the chance and pushed too far, but would ordinarily have been nice if appeased with love.
TBC Ashfur: Obsessive villain, nothing would ever have been good enough to appease him.

Thrushpelt, Bumblestripe, Leafpool, Sunbeam, Bristlefrost, and many others didn't do what Ashfur did when spurned by their loves.

Fanon Bumblestripe vs Canon Bumblestripe moment

Same! Honestly, the only things that would've made the Ashfur part of TBC even more great is if Hollyleaf and Yellowfang had made actual appearances—Hollyleaf for obvious reasons, but Yellowfang so she could've actually gotten called out for her "he only loved too much" excuse; I know the website addresses it, but it would've been nice to have seen something similar happen in-universe as well. Instead, we got DotC cameos. And I love DotC, but really.

"Why are you all so full of yourselves? ThunderClan was just the same when I was alive. So smug. Just like you are now. You all think you're so loyal, but when Squirrelflight dumped me for Bramblestar, none of my Clanmates stood by me. Not one. But I showed them. I ruled them all. They did whatever I told them to do. They exiled their Clanmates and fought each other, just to please me. And yet Squirrelflight still didn't appreciate how powerful I was. She acted like I was an ordinary warrior. She dismissed me like an apprentice. But I didn't give up. I brought her here. I thought we could be mates at last. I thought we could rule the Dark Forest together. [...] To be with me, of course. But she only wanted Bramblestar. [...] I'm going to destroy more than the Clans. I'm going to destroy the Dark Forest and StarClan along with it. StarClan can't survive without the Dark Forest. This place is no more than a miserable reflection of StarClan, and as it vanishes, so will StarClan. I bet it's already started. The fog will be swallowing up their precious hunting grounds. They have so much territory, they probably haven't even noticed yet—but, when they do, they won't be able to stop it. Once this place is gone. StarClan won't be far behind. It won't be long until they've all disappeared. Once the Dark Forest is gone, I'll be free to return to the lake. I'll make Squirrelflight suffer. She'll pay for betraying me, along with every cat she's ever cared about. Eventually she'll beg me to stop and become my mate. I wonder how many cats will have to suffer before she—"
— Ashfur to Rootspring while in the Dark Forest in A Light in the Mist, page 108-111

This (long-ass) speech by Ashfur himself proves that he literally pulled all of the shit he did because of his beyond messed up twisted "love" for obsession with Squirrelflight.

his ego from trying to coddle, baby, and control squirrelflight and have her all to himself was so fragile that of course he would be the type to snap and go all incel and evil over it. i wasnt surprised.

Hey! Thanks for taking the time to respond to my post I don't have any friends who read warriors so I've never had anyone to bounce these ideas off of, and it's really great to get to read other perspectives on things like this.

My interpretation of Squirrelflight's relationship with Ashfur was that she was hoping all along that things would work out with Bramblestar, which is fair, but I feel like you should probably make a choice between moving on and waiting to see what happens before you involve other people(cats?) in things, because it runs a high risk of hurting someone when you begin a relationship with one individual while pining for another. While I think this was a flaw in Squirrelflight's judgement, my real issue is with the series of events it sets off.

I listen to true crime on a regular basis and it's something I really enjoy, so I'm familiar with how people can react to situations like the one Ashfur was in. But in situations like that irl, It's usually a very targeted attack on friends, family, or the individual, which is what I meant when I said Ashfur attempting to kill The Three "made sense", because that tracks with real world scenarios and is (unfortunately) a reaction that can be observed in reality.
Where I take issue with it is when Ashfur sets out to destroy the afterlife and destroy all of the living clans. When you look at real world events, people who cause destruction and death on this massive of a scale are almost always motivated my manifestos and belief systems (think Hitler, Stalin, etc.) or hunger for power. To me, unrequited love seems like an incredibly unrealistic (and weak) motive for destruction on this massive of a scale. Which is why it seems to me that this role would've been better filled by a Tigerfirst, a "charismatic" leader who was motivated by a desire to create a new empire and install a new belief system.

By the way, I really like your theory that he developed an ego after avenging his parents. I could definitely see that being the case and playing a role in his development and eventual downfall. I also agree that his resentment of Graystripe makes sense in the context. And I never thought of Bramblestar as being the son of Ashfur's mother's murderer but that's a really cool perspective because it adds a layer to the whole craziness of the love triangle.

i'm not sure, didn't vicky say the fire scene was one of the first scenes she came up with? i might be misremembering though.