His motives get even more insane later on after all like:
-Should he kill said monster who is now a little girl?
and
-Everyone around me is lying, I can't trust anyone
However, this is Daisuke Moriyama we're talking about. He's done two manga titles so far and Chrono Crusade was just pure perfection. At least there's explanations behind it.
In terms of emotionally unstable characters, I guess Chrona from Soul Eater could count but then again, we got the background for said character a while ago, so their insanity is... Acceptable.