All y'all being like "no don't kill Migen his sorcery is gone so he's not a threat/it'd be more of a punishment to make him live with no magic" when, uh, he couldn't use his magic when he tore out Melissa's throat, so he's still definitely dangerous. I think removing his fangs and claws would be a good start, but you'd have to like, break his legs or something to keep him from just choking someone out. And that's not even getting into the whole "Yuri's still trapped in the magic box" thing
I kinda want Yuri to be the one to kill him, though, but if Nine does it I won't be disappointed or anything. Ultimately Nine is doing it in defense of someone else, and not for funsies like Migen. If Melissa's all like "no.......don't do it Nine.........." then that won't be disappointing either, though. At this point, Nine is killing Migen for Melissa's sake, not his own. So if Melissa is like "don't," then obviously Nine shouldn't. If Nine was going to kill Migen as payback for all the abuse, then Melissa stopping him would feel unsatisfying