[QUOTE=Kosh Naranek;7345843 However, I enjoyed it to no end. Not everything is right upstairs for Rick.[/QUOTE]
I don't think he's insane, but everytime he kills, his conscience still troubles him about what he had to do to Shane (despite it being a reaction to Shane leading him into a would-be death trap).
Before the zombie outbreak, I think it's fair to think if Rick ever killed anyone, he would be able to place that in the compartment of his job--it was not "out of line," or morally questionable based on how the character was presented early on. At present, he feels as if he's still in the wrong about Shane, which makes him a stronger moral force, as he cannot justify killing a friend--even one who wanted to kill him.