The Rambam wrote in chapter 1, law 1 of the Laws of Murder
Anyone who kills a Jew transgresses a negative commandment, as it says, "Do not murder". And, if he did it with malice in front of witnesses he is killed with the sword.The commentary here suggests that the prohibition of "Do Not Murder" related only between Jews, and not for a Jew who kills a non-Jew even if the non-Jew is from one of the righteous nations of the world. Also, in the book Fearers of God it's written that killing a non-Jew is not considered a transgression of the commandment "Do Not Murder" at all. And so, the Minchat Chinuch wrote:
One should not kill a single person, as it says, "Do not murder"-...but the killing of a non-Jew and even a righteous inhabitant of the Land of Israel that has accepted upon himself the Seven Commandments of Noah's Offspring are not included in this prohibitionFrom what we currently have in our hands and understand about the verse "Do Not Murder" it's impossible to learn the prohibition of killing a non-Jew.