Steve Rogers is responsible for trillions of deaths

Meh. There's a big difference between killing people directly (Thanos) vs. people getting indirectly killed because of an action you took (Starlord) vs. people getting indirectly killed because of possible inaction on your part (Captain America). They don't all contain the same amount of responsibility for the deaths of the people affected. For example, I know of no law that we could prosecute Cap for not murdering Vision.