Thinking through the Green Knight, his nature, his challenge
The green knight himself had no real fear of death, while requiring it of the person he was testing.
If that were true, the whole matter would merely be a monstrous imposition. It turns out, though, that it doesn't make a lot of sense to view it that way.
OTOH, Gawain may think that by beheading his challenger that he escapes the reprisal. Which may be part of the test itself. He wasn't commanded to kill the green knight, only to smite him as he chose.
We should, then, think of the green knight not AS a person, but simply as a test of honor, which Gawain ultimately faced up to, regardless of the consequence, leading him perhaps or perhaps not to his death, but at least his honor restored.