הלכה על בבא קמא 237:18

And the opinion of Rambam, may his memory be blessed, that he wrote (Mishneh Torah, Laws of One Who Injures a Person or Property 8:10-11) about the law of an informer is thus: "Once he said, 'Behold, I am informing about the body or money of x' - and even if it is negligent money - behold, this one has permitted himself for death. And we warn him and say to him, 'Do not inform.' If he was brazen-faced and said, 'No, rather I will inform' - it is a commandment to kill him, and whoever is first to kill him, merits. [If] the informer did that which he plotted and informed, it appears to me that it is forbidden to kill him; unless he is established as an informer, lest he inform [on] others." To here is the language of the rabbi. He required a warning for one who is not established as an informer and that he accept the warning. And for the one who is established as an informer, it appears from his words, that he does not need a warning. And an informer may not say, "Because x was afflicting me, I am informing [on] him to the gentiles" - as this does not exempt him from his punishment. But it is permitted for the community to inform to the gentiles about one who is afflicting the community. And so did Rambam, may his memory be blessed, write (Mishneh Torah, Laws of One Who Injures a Person or Property 8:11). And it is forbidden to destroy the money of an informer, on account of [An evildoer] "Prepares but the righteous one wears" (Job 27:17). And so is it concluded in the chapter [entitled] HaGozel (Bava Kamma 119a).