Splashtail

Personally, I don't think Splashtail killed Reedwhisker. Or at least not that he was/is the ringleader of whatever group of RiverClan cats is behind the planning of Reedwhisker's death, if that makes sense. But perhaps he might've gotten told to kill him by someone else and ever since then has gotten a taste for murder or to have more power in that group of cats as well as RiverClan as a whole.

Of course, his involvement to this extent is pure speculation on my part for now. Although I'm truly convinced at this point that Splashtail will indeed be revealed to have had at least something to do with the conspiracy against RiverClan's former leadership. At the very least, he is certainly a suspicious character at the moment.