Since HKBH wants him dead anyway, is he allowed to fulfill God’s will by taking his own life?
If he is commiting suicide because he is unhappy with his life, then the answer is no.
If he is a perfect Amaleiki goy, keeps all the sheva mitzvos bnei noach etc. and understands that it’s G-d’s will that he should be killed by Jews, then the answer is still no.
Probably, it is better for the person in the above situation to ask a jew to kill him. Then I think it would be ok.
But first, he must really be sure that he is indeed Amaleiki.