The kind of "killer" who commited homicide in the defense of the planet and it's 6 billion inhabitants from a man who had said and demonstrated several times through the movie what his intentions were? Convenient of you to brush Zod's decisions and actions under the rug and attack Superman for not saving everyone in a battle he was outnumbered and outmatched in. Zod was a sane individual capable of making independent choices. He CHOSE to colonize Earth and atop the rubble found New Krypton. Zod should be held responsible for his actions and decisions. No one twisted his arm to attack humanity or that family which finally pushed Superman in to the uncomfortable spot of having to use lethal force. Zod could've quit anytime and surrendered peacefully, but he didn't.
Krypton did have it's chance. The Council on Krypton doomed their world by mining the core for resources, making the planet unstable. To rub salt in the wound, the council didn't lift a finger to evacuate their planet despite it falling apart around them and provocations from Jor-El and Zod of the oncoming danger. The government of Krypton doomed it's inhabitants. It's wrong for Zod to wipe out humanity just to rebuild a dead world, with the same flawed ideas that saw Krypton destroyed the first time.
Watch the movie again. The lion share of destruction is cause by the world engine. Which cascaded out a few blocks in the city. Superman and Zod crash through 8 or so buildings during their final fight. All of which were empty; excluding the last one where Zod died. So the "gross recklessness" charge is false. The World Engine was wrecking the planet while Supes was on the other side of the world trying to stop the machine creating the gravity pulses so the military could execute the plan to trap the Kryptonians back in the phantom zone. This is all explained via exposition in the movie.
No the writers did not overthink or paint themselves in to a corner. When Zod and his followers attack Earth, there is little the inhabitants can do to resist.
Need proof?
Watch
Superman II by Richard Donner and Richard Lester. See how Zod and 2 other Kryptonians conquered Earth in under a day.
Read
The Supergirl Saga by John Byrne (Superman #21, Adventures of Superman #444, and Super #22). Where Zod and 2 other Kryptonians wreck Earth unopposed since that world's Superman had died 10 years earlier.
Here's a pic of their conquest.
Read
Superman Last Son by Geoff Johns and Richard Donner (Superman 1978 and Superman II). In it Zod and his followers wreck Superman, the entire JLA and JSA IN ONE FRAKKING DAY. More photos of humans trying to resist after the JLA and JSA had been captured.
MOS did not set some precedent for mass destruction or killing in Superman comics. It's been there for years. See: Death and Return of Superman, Superman Red Class, Superman Birthright, War of the Supermen, New52 JLA vol 1, Superman Earth One vol 1, the aforementioned Supergirl Saga, and Last Son.
MOS being a film based on adaptation had 75 years worth of material to draw from. Since they did an alien invasion movie. There is nothing in the film that has not been in print and published in a Superman comic beforehand. That's the hard truth people don't want to hear.
^Superman Red Glass 1991