There is no clearcut evidence in Superman II that anyone died at the end of the theatrical release of the film. although I admit it might be implied.
"but Superman throws him against a wall and lets him fall, presumably to his death. Non leaps to attack Superman and falls in the same pit. Lois punches Ursa, who also falls into the pit."
Yes, PRESUMABLY to their death(s). But we did not see a pit full of corpses, all we saw was Superman's conversation with Lex Luthor and Lois Lane, then it cuts away to him flying her back to civilization. Since it seems that it wasn't nearly as cold inside the fortress of solitude (no one was wearing winter coats or anything), one could also presume that Superman merely trapped them in the pit and came back later to take them to jail.
Furthermore, in other releases of the film and deleted scenes added to various viewings, the fate of the criminals is revealed. In one version they are arrested, and in another version Superman turns back time which re-imprisoned them in the phantom zone.
In 1984, when Superman II premiered on television, 24 minutes were re-inserted into the film (17 minutes on ABC). Much of the extra footage was directed by Richard Donner. In the ABC-TV version, a U.S. "polar patrol" is shown picking up the three Kryptonians and Lex Luthor at the end of the film. Without this ending, it appears that Superman has let the Kryptonians die, though Superman has a strict code against killing and their deaths aren't necessary once they are depowered. On the other hand, the theatrical version's ending implies that Luthor is left stranded at the Fortress of Solitude, leaving the viewer to wonder how he got to prison in Superman IV: The Quest for Peace — that question was answered in the extended versions where Superman reverses the rotation of the Earth where one of the things he does involves preventing Lex Luthor from escaping from prison. The ending of the extended cuts also has Superman, with Lois standing beside him, destroying the Fortress of Solitude.