@TLIK
1. Killing any person is murder.
2. The Torah prescribes a code of conduct for non-Jews. Whether the non-Jews choose to follow the Halacha or not is irrelevant. The question in the OP was what the Halacha says about this case, which is an interesting question.
@lakewhut
Even if you witness a murder you may not kill the perpetrator, even though moments before he was a rodef.
As others pointed out, even in the case of a rodef, if יכול להצילו באחד מאיבריו and you killed him, you’re the murderer.