The crucial point to remember is that Erfworld runs according to game rules not real life. Game rules are an abstraction of real life, designed to make the game playable, and are not necessarily realistic.
Sizemore would break if he knew the "rules" of the world Parson comes from, and the wars that had been fought in it.
Originally Posted by Midnight Roamer
I think he did the only morally acceptable thing by killing everyone.