I was listeing to the ABC Evening news and the news reporter said “What he thought was a confidant turned out to be a government snitch.”
OK it gets the point across clear and with no ambiguity but I was wondering if there was a more “proper” for lack of a better word, than “government snitch.” I was thinking infomer, but that doesn’t seem quite the same
Confidential informant, or CI.
In the case of a low-level guy who makes a deal to squeal on the bigger guy, the usual term is Cooperating Witness.
They’re referred to as CI’s - confusingly I’ve heard the acronym expanded to either “Confidential Informant” or “Criminal Informant”. The former makes a lot more sense to me but I’m not certain which one is technically right.
not exactly what you’re looking for, but as an aside there is also the related phrase “turned state’s evidence”. It generally refers to people who themselves have done something illegal, or been a part of something illegal, but then rat out their co conspirators in exchange for clemency
I’ve heard of CIs, but the Cleveland Police Department routinely refers to them as CRIs - Confidential Reliable Informants!
That’s because they never use plain old “Confidential Informants” when they’re asking you issue a warrant - they only use Reliable ones. Nothing but the best!
They only use their low-grade “confidential informants” for things like parking tickets.
I’m sure there was no real answer, but the word “snitch” sounded way to childlike or immature to be used by ABC News. 
In some contexts, you can call them whistleblowers.
“Informer” was the traditional term in Ireland, since at least the nineteenth century.
I thought it was “approver”.
I thought it was “grass”.
“Source” and “asset” are used on the intellgence side.