It always amazes me to see the view that Superman killed Zod in cold blood at the end of Superman II.
I know this is just anecdotal, but none of my friend group who saw the movie when it opened came away with the idea that Superman had killed him or let any of the other Phantom Zoners die. There is nothing that establishes the Fortress of Solitude is built over a bottomless pit, or any other kind of chasm. All we see on screen is that a) Superman arrived at the Fortress before the villains and set up a trap for them and b) when the trap is sprung they vanish in a mist. That's it.
The idea that Superman killed them goes against the very tone of the movie itself, especially in its depiction of Superman, and is speculation that is not supported by the movie itself.