I agree that is more likely, but that scene is not on my Superman II dvd. So that is why I assumed that he killed Zod.
One of the first rules of comic-book physics is that if an arch-villain seemingly falls to their death, they'll inevitably survive, usually without any explanation. There's no counting the number of times in Batman comics over the decades, both in the early '40s and post-'70s, when the Joker apparently fell to his death only to reappear perfectly intact later on. The same went for Hugo Strange, the other primary recurring Batman villain in the early '40s. The Joker of
Batman: The Animated Series had the same ability to return from seemingly certain death, though not specifically falling. In
Batman '66, Catwoman returned after falling to her apparent death on two separate occasions -- and
Batman Returns made her "nine lives" resurrection ability explicit.
And it's not limited to comics arch-villains -- other examples include
Doctor Who's Master and the original
MacGyver's Murdoc, though their apparent deaths rarely involve falling (I think at least one of Murdoc's "deaths" did, though). It's a routine way to give a hero a seemingly certain victory over a villain while leaving the door open for that villain to return later on. It's basic storytelling logic: If there's no body, there's no proof they died. So I see no reason to assume anything different about
Superman II.
Of course, you could still argue that Superman intended to kill Zod et al., but that would be out of character. If we stipulate a universe where a fall from such a height is potentially survivable, then it follows, by the logic of vintage Superman stories, that Superman
knew the fall was survivable and intended it to be so.
Frankly, what's far more out of character for Superman in that movie is the bit where he goes back as Clark and beats up the bully with his powers restored. That's the kind of abuse of power over a weaker person that Superman would never consider in a million years.
