How would it be *murder* ? If Zod was out cold and he stamped on his head killing him you would have s point but the law generally doesn't consider it murder if you are acting in self defence or the defence of others.
Pretend I said "justifiable homicide" instead.
Anyway, it turns out I spoke in error anyway. Murder is "the act of killing another human with malice aforethought"
, so it's actually impossible for Superman to murder Zod.