doomguy stops a demon invasion brough on by a capitalists greed. masterchief fights in an alien war without question, probably for space oil or something
Actually, Master Chief and the Spartan program wasn't started to fight in the alien war.
It was originally an effort to suppress rebellion against the UNSC. Master Chief fights for the government that kinapped and brainwashed him into being a child soldier.
Realistically the Spartans should defect to the covenant out of pure spite. They have more in common with elites than they do humans anyway.
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