Superman is not a killer, however capable he is of the deed, it's never his intention. In general Zod's case, he knew no other way to stop him but to kill him once he had him in the position to break his neck. Superman's evolutioin on earth had rendered him more powerful than Zod, but not as skilled of a fighter. Which after seeing no end to the constant bludgeoning between them, it left him with no other choice...and this is why he cried out after do so.

Superman is not a killer, however capable he is of the deed, it's never his intention. In general Zod's case, he knew no other way to stop him but to kill him once he had him in the position to break his neck. Superman's evolution on earth had rendered him more powerful than Zod, but not as skilled of a fighter. After seeing no end to the constant bludgeoning between them, it left him with no other choice...and this is why he cried out after doing so.