I came across this article about informants breaking the law. There has been a lot of discussion relative to this matter as a result of the Whitey Bulger case.

The article is a few weeks old but does provide some good information. As you can see, it's difficult to get exact facts.

Sorry, I forgot to include the link when I first posted.

http://www.wbir.com/news/article/283170/16/FBI-allowed-informants-to-commit-5600-crimes

Greg Scarpa was an informant for the FBI while also continuing a life of crime. Same for Sammy Gravano

Mark rossetti can now be put on the list, but i think its just as bad when they just let people like gravano and martarano get off with minimal time when they have murder counts in the double digits.

Yeah, I know what you're saying. I'm sure there's a lot of discussion / debate about giving lighter sentences to those willing to "rat-out" their friends in exchange for a much lighter sentence. It doesn't seem fair, but I imagine it may be the only way to get some of the "bigger fish." I've talked with some local law enforcement officials and that's the only explanation they gave. But it does seem to be unfair because some of these informers have committed numerous murders, just like you pointed out.

Tough decision for law enforcement.

This happens all the time. It is ignored unless the case is a high profile case, they might be able to impeach the witness but the system is so corrupt it's a joke.