They probably figured he was sad enough about everything else. Honestly even if we all agree that killing isn't right, Zod had it coming. It was absolutely like a dude holding a gun to someone's head in front of a cop; you just don't really feel sorry for a sane person doing that. In the moment he basically asked for it so I'm glad that by BvS he wasn't having nightmares or anything.
The idea of "do something else" always haunts the narrative though. The point sometimes is what actually happens and "something else" can't cut it.