Zod and his merry gang arrive on earth to get Kal-El, I get the impression that he would have gone quietly if they had left Earth alone. However they then proceed to try to terraform (technically kryptoform) the Earth.
Wouldn't it make everyone's life a bit easier if they simply chose another planet, why didn't they do this?
If you remember Jor-El's dissertation:
The Kryptonians expanded into the universe and had established outposts on or near other planets in a plan to conquer those worlds. Earth was one of those worlds 20,000 years ago. They had gone so far as to establish a base on the dark side of the Moon. Inexplicably something undermined the project and the Kryptonians died (no further explanation on was given).
Though it went unsaid it was implied...
The establishment of the base on the Moon meant the Kryptonians had planned to take over the Earth before Humanity had achieved any significant level of technological development and alter the environment for the Kryptonians. Earth being a life-bearing planet already would have been an excellent biological base to alter the environment for the Kryptonians to establish their new ecosystem.
As to the World Engine: scattered notes indicate it was found on Daxam, which apparently in the DCEU does not have live descendants of Krypton living on it, unlike the comic universe, which does.
Settlers landed on the planet and used a World Engine to terraform the planet's environment to their needs. Sometime later, the outposts were abandoned and Daxam's inhabitants quickly perished after running out of resources.
Shortly after Krypton was destroyed, the surviving members of the Sword of Rao (General Zod's ship) came across what was once Daxam, took what they could find and set out to locate a planet they could turn into a new Krypton. --DCEU Wiki
They needed the Codex that supes had in his blood (as well as the scout ship left behind on Earth) to recreate the Kryptonians. So they had to go to Earth anyways before they could terraform any planet. Prolly along the lines of: I'm already here, might as well anyway.
Considering the fact that Kryptonians have left an outpost for colonization on Earth we can assume that not every world can become a candidate for terraformation. Zod said to Kal El that new foundations has to be built on something, even Jor El understood that. Also, Zod's visited worlds pretty much were destroyed beyond repair. So in the end chances of finding other suitable worlds are extremely small and that's something Zod has no luxury to even consider practical. Also, flaura and fauna would in time adapt to the cataclismic change of gravity and atmosphere changed composition, while humans would not. That being said, Zod is a warrior and a conquerer, not a diplomat or a scientist, his atitude is preservation of Kryptonians at any cost. Terraformation of Earth seems not only the most practical but the only possible option to him.
It's an assumption that the World Engines have the capacity to terraform planets to that degree. By the time we see Krypton's civilization at its end there are no sister planets, no colonies, and nowhere to go. This strongly implies that World Engines could not transform any location of their choosing to be Kryptonian habitable. When Zod and his crew are shown at a dead colony, the environment isn't terraformed despite possessing a completely functional World Engine (which Zod commandeers).
Therefore, the easiest explanation for why Zod did not simply select another planet was because he couldn't. It was beyond the World Engine's capability and his search for a new suitable environment was unlikely to be more successful than the Great Age of Expansion with the entire planet behind such process. Zod's only advantage was the Phantom Drive, however, we cannot presume that either the Phantom Drive or the World Engine are infinitely usable resources. Quite to contrary. If Krypton's resource issues could have been resolved by unlimited consequence-free energy, the planet would not have been driven to collapse. Thus, it is plausible that both the Phantom Drive and the World Engine are finite resources.
That being the case, Zod wouldn't arbitrarily or inefficiently attempt to terraform that which is unlikely or unable to sustain Kryptonian life... it would have to be a location that met all the necessary specifications for a new civilization.
All Kryptonians are genetically engineered for a purpose. The warrior Zod was genetically programmed to ensure the existence & continuance of Kryptonian civilizations & populations, regardless of the methods and costs. Humans are not Kryptonians & Earth is not Krypton. In the same way Europeans colonized much of our world, it was his intention to neutralize the indigenous/aboriginals and recreate the surroundings to mimic that of his native land. Kind of like James Cameron's AVATAR. That being said, why not terraform Mars instead?
Well from what I can tell, Kryptonians killed each other on that outposts and probably destroyed also the atmosphere so do the entire planet. The Earth was the nearest habitable planet around and it also possesses the colonizing ship. Terraform the dead planet like Mars would require more energy and materials. Colonizing Earth was their best and only chance.
I think that, like humans, Kryptonians need a circumstellar habitable zone to survive, thus ruling out Mars or any other planet in our solar system. And it makes sense that they would want to colonize Earth, given the extraordinary powers that it's yellow sun and lower gravity afford them. On the other hand, I really hated how "one-note" of a villain Zod was. His motivation to overthrow Krypton's Science Council never made sense, seeing as he seemed to agree with their policies about artificial population control. It would have made for far better storytelling if Zod was more in line with Jor-El's way of thinking (especially since he really seemed to regret killing him) and wanted Kal-El's help finding another planet to colonize. Then, it could be someone like Jax-Ur who goes behind the general's back and starts terraforming Earth. Or it could be as simple as Zod and his followers have become embittered by their time in the Phantom Zone and therefore stopped caring about the morality of their actions.
This is in my opinion just poor writing. Zod released the world engine only when he was told he could extract the Codex from Superman even after his death. He released the engine as a way to take away his powers and kill him.
If superman would have been a little bit smart, he just would have reach an agreement and surrender the codex after terraforming mars.
Jor-El explained that the sun was brighter and gives more strength to the Kryptonians. I think because of this Zod wanted to have the new strength the sun gave them but in order to adapt, without the pain, the Earth would need to terraformed to Krytonian needs. And also, like Zod said, Earth had the perfect foundation to rebuild a new Krypton.
