It is forbidden to inform about a colleague (Jew) to the gentiles and endanger his physical person or his property. This applies even when the person concerned is a wicked person who commits sins, and even if he causes one irritation and discomfort. […] It is permitted to kill him before he informs. When he says: “I will inform on so and so and endanger his person and/or his property” - even property of minimal value - he has made it permissible for others to kill him.

Mishneh Torah, One Who Injures a Person or Property 9:10