Some may see Carl's decision as correct (considering the active war situation), but his conclusions are generated by a take-no-prisoners mentality that will only lead to pain, as he has to realize others (like the Governor) will see the rest of the Atlanta/farm group in the same way. He's quite dark.
That's Carl's after-the-fact attempt at a rationalization for what he did so that he could justify it to himself.
If the kid had dropped his weapon and followed orders, I doubt Carl would have shot him. It was all about the threatening actions the kid was making while "surrendering" that caused him to pull the trigger, and it was the right choice giving the situation.
Carl's not acting like a psychopath at all. Hardened, yes. A realist, yes. But not psychotic at all, not given the situation he's in. What Rick did with the hitchhiker is far worse than anything Carl's done so far.