It's the weirdest frakking thing. Grant Morrisson and Mark Waid took issue with the level of destruction and Superman killing in MOS. But in later interviews they said the level of action in MOS was fine but the collateral damage in civilian areas is what bothered them. So it's a matter of venue that rustled their jimmies. Superman and Zod fighting in a city is wrong, but Superman wailing on Zod from Earth, Mars and all the way to Pluto is ok.
Also with the scenario Morrisson outlined, it makes it seem like he's ok with the villain dying, just as long as Superman doesn't kill him. Idk about you but dying of asphyxiation in space would be an ugly way to go. I don't see how Superman comes away more of a hero allowing Zod to suffocate in the vaccum of space than Superman choosing to break Zod's neck in the defense of that family and all of humanity.