Only because that is the way he was written for the movie. You get that Snyder wrote that, right. The backstory is written to create the specific ending we got. Rewriting is not just writing one scene, but everything related to that scene.
What are you pitching here? Do you want a less menacing, less intense general Zod? Zod is always a criminal on Krypton and renegade actor before he ever even meets Superman.
Zod was engaged in an unseen, treasonous coup in the 1978 movie and a treasonous coup in MOS that we did see. The futility of both actions is Zod launches these insurrections on the last day of the planet. In Zod’s mind, in many instances. He is the hero of the narrative.
Remember that Zod is meant to be a Hitler stand-in. The dark mirror of Superman. An evil alien Übermensch. How would you propose to soften his image that he could battle Superman, lose and still be ammenable to leaving Earth peacefully. Like you suggested?
There are enough stories out there where Zod and Superman have worked together temporarily. Zod, in the comics, eventually gets his own new Krypton.
And? There are stories where Superman has worked with Darkseid, Brainiac and Luthor temporarily. They are all unrepentant mass murderer before and after that alliance.
You should read the history of Zods list in my previous post in the spoiler tag. DC is not interested in changing Zod into some hypothetical “uneasy ally” that you seem to be suggesting. Superman has one of those already. That would be Lobo.
I think you are conflating something you don’t like (Snyder’s take on Zod), with something you think is wrong. See the aforementioned list. Zod is a recurring ‘one and done’ villain in the stories he appears in. With many of these oneshot appearances involving Superman or one of his allies having to swing the sword to put him down.
Zod is not Luthor. There is no conventional prison that can hold him. And seemingly, most writers have concluded that just putting Zod back in the zone after he wreaks murder & mayhem is unacceptable. Probably something to do with maintaining the effectiveness of the Phantom Zone as a “perfect prison” in the narrative. A prison that the same guy keeps breaking out of. They’d rather put him in the ground, than write a scenario that sees him be put back into the PZ.
P.S.
The New Krypton that Zod ruled (along with Kara’s mother) was obtained after an arc where Superman rescued the bottle city of Kandor from Brainiac. Superman, Alura Zor-El, and scientist who reversed Brainiac’s shrink ray did the work. Alura was feuding with her nephew at the time and released Zod from the PZ to help her oppose Clark and the forces of Earth.
That’s a lot of hoops to jump through.
I hate talking about that story. It happened back in 2008-2010 and ended abysmally.