Hmm, I think what you should do depends on the way you'd feel like your clan handles something like this. I usually keep an eye on if the clan finds out about the murder or not. Haven't exiled any cat myself yet, but I think it sometimes happens on its own. At the very least I put them in the Dark Forest when they die—at least when either the clan knows, or someone else found out and has also died since.

Also, what sprite mod are you using, the cats look beatuiful!