MESIRA - Rabbinic Law Unto Themselves

Mesirah (or mesira, Hebrew: to turn over‎) is the action in which one Jew reports the conduct of another Jew to a non-rabbinic authority in a manner and under the circumstances forbidden by rabbinic Law. This may not necessarily apply to reporting legitimate crimes to responsible authority, but does apply to turning over a Jew to an abusive authority, or to a legitimate one who would punish the criminal in ways seen as excessive by Jewish community, though "excessive" punishment by non-Jews may be permissible if a precept of the Torah has been violated.

The term for an individual who commits mesirah is moser (HE:מוסר) or mossur. A person who repeatedly violates this law by informing on his fellow Jews is considered subject to "Din Moser" (law of the informer), which is analogous to "Din rodef" in that both prescribe death for the offender, and according to some, in some circumstances he may be killed without warning.
The source of the ban comes from the Bava Kamma (HE:בבא קמא) section of the Babylonian Talmud. Probably the law was instigated to ease Jewish life under Roman or Persian rule. This law is discussed in Babylonian Talmud, by Rambam and in Shulchan Aruch. Shulchan Aruch, however, states that in the cases that if Jews do not testify against other Jews in the gentile court, it will be obvious that Jews are covering up for each other, they should testify.
Maimonides states:

Whoever adjudicates in a non-Jewish court ... is wicked and it is as though he has reviled, blasphemed and rebelled against the law of Moses.

Maimonides further explains: "It is forbidden to hand over a Jew to the heathen, neither his person nor his goods, even if he is wicked and a sinner, even if he causes distress and pain to fellow-Jews. Whoever hands over a Jew to the heathen has no part in the next world. It is permitted to kill a moser (informant) wherever he is. It is even permitted to kill him before he has handed over (a fellow Jew)."

en.wikipedia.org/wiki/Mesirah