The prohibition of mesirah is well known among religious Jews. The severity of this issur becomes ever so clear when we read in Shulchan Aruch that a mosser is dino ke’akum with respect to writing a sefer Torah (Yoreh Deah 281:3) and with respect to shechitah ( Remah ibid 2:9). Even if the mosser is otherwise an observant individual, and is meticulous in fulfilling his religious duties, because he demonstrates his rejection of the unity of all of Klal Yisroel (by his act of mesirah), he is treated as an akum (see Rambam, Hilchos Teshuva 3:11 and Nefesh Horav pg. 235).

If, however, one is guilty of a crime, and according to the law of the land deserves a prison sentence, or will be put to death, even though according to Jewish law his punishment would not be as severe, this is not mesirah (see Ritva to Bava Metsia 83b; Dvar Avraham vol. I pg. 8). One would still not be allowed to hand this individual over to the civil authorities because this is the equivalent of returning aveidas akum, which is usually not allowed. In an instance of avoiding a chilul Hashem, just like we would be obligated to return the aveidas akum, so too we would be obligated to hand over this individual (see Rama, Choshen Mishpat 388:12).

From here. 

The Hebrew word is mesira, which means basically you are not allowed to be an informant," said Rabbi Shea Hecht, a well-known figure in Crown Heights.
"In essence, I am not allowed to snitch, period."  From here.


As society changes, halacha has also evolved. Many rules designed for a different era are essentially defunct, through loopholes or the like. For example, it used to be forbidden to drink wine with a gentile or to break bread with him. Now it's basically fine, as long as you drink mevushal wine and eat in a kosher establishment. It used to be forbidden to drink milk supervised by a gentile because we were afraid that milk from treife animals may be added in.  Now, in many countries, there are laws in place to prevent dairy manufacturers from adding anything extra into the product and cholov yisroel is a chumra.

The halacha of mesira also needs to be retired, in the same way as the rules mentioned above.
 
The laws of mesira were designed to prevent tyrannical and anti-semitic governments from harming innocent Jews.They were not designed to allow Jewish criminals to avoid punishment. Regardless of whether chillul hashem is involved or not, Jewish criminals only harm our communities and should be subjected to the justice system, just like anyone else.

Think about it. Do you really think that God wants Jews to be able to get away with crimes that everyone else would receive prison sentences for? Do you really think God expects botei dinim all over the world to handle the appropriate consequences for criminal activities in the frum world? Do you think cherem (excommunication) for a money launderer or a child molestor or a wife beater is enough?

I read these questions on imamother a lot. This and this happened to my child and I don't know what to do. The answer is always to call your local orthodox rabbi before speaking to secular authorities.

In most places, Jews do not live in countries with anti-semitic and tyrannical leadership. Jews have no less of a chance at a fair shake in the justice system than anybody else and there's really NO REASON TO CALL YOUR RAV when faced with a crime possibly committed by a frum person.

Just call the cops.

Mesirah is outdated and harms our community more than it benefits us.