One of the most objectionable acts a Jew can do to another Jew is to inform on him to the government.
This is most serious and can lose you Olam Habah:
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)."
When someone pleads guilty, he is telling the government that he did a crime. In exchange for this, he will benefit (reduced sentence).
Is it relevant that the Jew being informed upon is the same Jew that is doing the informing? After all, we know that one is not permitted to murder himself.
