Qitzur Shulchan Arukh – 183:4
If it prohibited to surrender on a Jew to the control of non-Jews, whether bodily or with respect to his money, whether through action or speech, to inform on him or to reveal his hiding place. Whomever surrenders [a fellow Jew] has no place in the World to Come. Even someone who is evil and a master of sin, it is prohibited to turn him over, neither his body or his money. Even if he is a troublemaker to him and causes him problems constantly with his words. However, if his friend turns him in, and there is no way to save himself except through turning in [the informer] it is permitted.
None of the above applies to eliminating someone who is dangerous to the community who can not be removed in some other way. In other words, while informing on a fellow Jew to non-Jewish authorities is prohibited, allowing a child molester (for example) to continue posing a danger to our children is a greater prohibition. If someone finds themselves in this situation, one must consult with communal leaders to see if the authorities are a necessary means of getting him off the streets, and if so, speak with rabbis about the permissible way to do so.