I have actually spent some time on this question and it seems a very complicated issue regarding schar for an Amaleiki who kills himself.
First of all, we find the gemara in kiddushin that says that non-jews do get rewarded for fulfilling one of our Taryag mitzvos. This is learnt from the story of Dama ben nesinah who was rewarded with a parah aduma for not waking up his father.
Also I heard that the Maharal says that chasidei umos haolam do recieve some form of Olam Habaah for fulfilling their mitzvos.
Regarding the issur for a goy to commit suicide, it is a machlokes achronim. The Minchas Chinuch says that since the issur suicide is not learnt from the pasuk “lo sirtzach” it doesn’t apply to goyim, since to be included in the “7 mitzvos” a mitzvah had to be repeated at sinai.
Others ask from a psiktah that says that “lo sirtzach” can be darshaned to mean “lo sisratzach” -don’t be killed (suicide).
The Rambam in milochim says that if an Amaleiki accepts the 7 mitzvos and agrees to be at peace with us then there is no mitzvah to kill him. Since this amaleiki is now killing himself, he is now at peace with us and he also will not be able to be ovver on any of his 7 mitzvos. This may count as an Amaleiki which there is no mitzvah to kill.
However regarding the mitzvah of establishing courts, he may still be liable if it was in his ability to do so while he was alive.
If we follow the shitah (see above) that suicide is assur for goyim, this should constitute a mitzvah haboah biaveirah. However according to the MC this Amaleiki would probably not be chayav to be killed since he was mikabel the 7 mitzvos and is not doing anything wrong now.
The Rambam in milochim 5′ 3′ says that “tshuvah” is only possible for jews. Therefore if this amaleiki was once ovver on one of his 7 mitzvos, he cannot do tshuvah and should be killed, not for being an amaleiki but for not keeping his 7 mitzvos. If this amaleiki was ever in his life ovver on one of his lavvin, he should be permitted to kill himself through “Hereg” (the misah for goyim). I don’t know if a goy who gets killed for his aveirah gets back his Olom habah and recieves reward for the mitzvos that he did.
To summarize- An amaleiki who kills himself way have done an aveirah and lost his Olam Habaah (????? ???? ???? ??? ?? ??? ????? ???) he may have done a mitzvah or it may just be allowed but not a mitzvah.
Comments welcome (lihagdil torah)