Because it's an old movie with plot holes. And on the second question, by giving in and killing him, he'd be giving into the dark side.
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So your saying that for the second one.... He cant kill one person but he can kill hundreds
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If he killed Vader, it'd be out of revenge and if he killed him while he was down, it wouldn't be in self defense.
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Had it been a stormtrooper in vaders place would he have killed him?
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Depends. He'd still be unable to fight back, so no.