I'm assuming you mean how he can be a companion of the Knight after what he did. First of all, him killing Revan and the Exile is player knowledge, your character has no idea about that when he meets him. But even if he knew, the Jedi in general believe that everyone can be forgiven and turned to the light. So even if your character knew, he would probably say "yeah, he's done some bad things, he's a Sith after all... but he's trying to do something good now and we should give him a chance."
As for the monster stuff, for me it was always a case of "doing what you have to do"... a tad drastic, no doubt, but from a Sith point of view, he did it because he had to, not to satisfy some sick pleasure of his. For a Sith, I'd say that counts as rational