Are you sure it’s Peak who did it? You should check the code before banishing him to the dark forest forever.
this question is a little silly to me lol, like i for sure understand where you're coming from, but you have to remember that these are just randomized data in a game, y'know? it's more about the story i create and what I believe than whatever might actually be presented. in reality, it's likely that Peakstem didn't kill anyone outside of Anchovybuzz (even then, that's not technically confirmed) and the murders/missing cats were all just completely random chance
i like to theorize who the murderer might be and collect the pieces that are there rather than just pop open the game's data and have the answer! (also i dont know how to do that)
besides, its not technically forever. you can unbanish DF cats whenever you want unless you have fading on! which i dont lol
if all of the actual murderers come forward and confess like Fishmask did for Ferretpaw's murder, then maybe i'd reconsider his banishment, but, for now, he definitely killed those guys lol