It's all purty simple, actually. Ya gotta be the more bigger man, see? Like, if some guy robs a bank, it would be stealin to take the money back from him. Stealin is a sin, so...
More moral cowardice, lack of resolve, and pansyism has been advocated in the name of "holiness" than any other excuse, best I can tell. When Pearl Harbor was bombed, we shoulda just turned Washington over the the Japanese. Otherwise, we might have to (gasp) kill some peoples.