...what I really dont understand is how kryptonians died of anything when they got to earth...but I think I saw some dead bodies in the arctic ship that colonists used across the galaxy...
They died
before arriving on Earth. This can be used as an explanation for why they didn't colonize Earth. Or you can use a Prime Directive like explanation given their reticence to execute criminals (Zod committed high treason and still isn't facing an explicit death penalty, only a
de facto one). Or you can use the Prequel comic. It's ultimately immaterial.
Ultimately they never reasonably explained why Zod knowing how fragile the codex was and the state of the future of krypton was....ignored that he was nigh unstoppable on earth as is...along with all his fellow kryptonians...many of which were warrior class. . . .
Why am i risking all that when I can do the codex and rebirth after i've subjected the planet??
Don't forget that General Zod was convicted and sentenced for attempting a coup... that is, betraying, rebelling, and being a traitor to the government he serves.
Those who are traitors have the reasonable tendency to fear betrayal themselves and worry about traitors in their midst.
This is reinforced by the fact Zod only trusted a total of two Kryptonians and himself to go to Earth and gain access to super powers. If Zod absolutely trusted his crew, there's no reason he couldn't have drop-shipped them all around the planet to enforce his plan.
It's clear that Zod
doesn't have that absolute trust of his crew. And for good reason. If someone in a lesser caste or rank like, say, Jax-Ur were to taste that god-like Superman-level power... do you see these
criminals cooperating with Zod and giving that up out of mere loyalty so that they can serve under
his rule?
In a powerless society, Zod's military caste and might can still control things by force. In a powered society, Zod is just another equal. Zod's whole problem with the Council was that they didn't follow his eugenic ideals, so I doubt he wants to give up that control when forming a new Krypton.