The gemara illustrates from the story of Rav and Rav Kahana that it is permitted to kill a person who threatens to be moser. The Shulchan Aruch (c.m. 388:10) writes that this applies even nowadays because a moser has status of a rodef since it will lead to physical danger (as the sm"a explains, when the goyim find out he has money they will threaten him physically). The Rama rules that prior to killing him, he should be warned, but if there is no time to warn, or injur him as you would stop a rodef, you can kill him on the spot as r' kahana did (the yam shel shlomo discusses why r' kahana needed to run away to do teshuva if he was permitted to kill the moser. He suggests that either he needed teshuva for paskening in front of his rebbi, or he needed teshuva for endangering his own life). This applies only until the moser actually commits the act, but once he has already reported to the goyim, the shulchan aruch (388:11) rules that it is assur to kill him, just as a rodef can't be killed after taking someones life (unless there is a chance that he will continue). The shach points out that the ri"f seems to disagree with the rambam and shulchan aruch, that even afterwards he can be killed. Even according to the shulchan aruch that once he did the mesira he cannot be murdered, if he was repeatedly moser, the Rama (15) writes that he can be killed through grama. The Shach quotes the teshuvos ha'rosh that included in "grama" would be hiring a hit man to kill him. The maharshal disagrees and holds that hiring a hit man is the equivalent to killing with his own hands which is not allowed unless there is imminent danger of him reporting again. The rationale of the maharshal is that by a goy we say יש שליח לדבר עבירה to be machmir, so it is as if the jew who kills through a non-jewish agent actually committed the murder himself.
R' akiva eiger refers to a teshuva of the beis yakov (siman 2) who says that even if we accept that a goy can be a shliach for a jew l'chumra, since a goy is also forbidden to kill we should say אין שליח לדבר עבירה so that the act of murder is not attributed to the jew at all? The Beis Yakov explains the opinion of the maharshal based on the tashbetz who says that when it is obvious to the sender that the shliach completely disregards dinei torah, he cannot have a claim of דברי הרב ודברי התלמיד דברי מי שומעין which is the basis for אין שליח לדבר עבירה. Therefore, by a goy and even a yisroel mumar we would say יש שליח לדבר עבירה to consider the act of the aveira as if it were performed by the sender himself (he continues to suggest that the concept of יש שליח לדבר עבירה when the shliach is a goy, only applies when the sender is a jew, but when the sender is also a goy we would say אין שליח לדבר עבירה).