It can be answered with exactly that.
Zod’s death doesn’t work because it is neither led up to so that it lands nor is it followed up in any capacity whatsoever. The movie breezes past like it, and the third act itself, didn’t take place. There’s zero thematic or underlying story reason why he kills Zod other than “because it’d be cool” and that about sums Snyder up. Zod’s death was botched in Man of Steel, and any argument that goes “but Donner” is engaging in what-about-isms that I have no interest in, mainly because I don’t particularly love the Donner films either.
We’ve had that debate before, but it isn’t my point which is this: I’ll concede that there are people out there with double standards who judge these movies unfairly as long as we stop pretending they represent the majority of us who responded negatively to these movies.