Aside from the biblical prohibition of killing a non-Jew because of the prohibition of 'spilling the blood of another person'- it's appropriate to mention the words of the Sages in Tanna Divei Eliyahu on the stringency of killing a non-Jew. [One should not kill a non-Jew] because it is a desecration of God's name, and the fate of a person who kills a non-Jew is that ultimately he will kill a Jew.
...The end of one who kills a non-Jew is that he will kill a Jew. All of this is ultimately done to sanctify His Great Name.From this a number of obligatory laws can be extracted. One should not even harm a non-Jew (even those who desecrate the Seven Laws of Noah, simply because they deserve the death penalty, which will be clarified in chapter 2) in order to prevent a desecration of God's name. These laws are stated in order to prevent loathing and for the sake of peace (between Jews and non-Jews). And, certainly in matters that pertain to 'killing a non-Jew', one must consider if such grounds are even realistic (the era, country of residence, etc).