It should surprise nobody that a narc, who doesn't have the spine to take responsibility for his or her own actions, has decided to take the easy way out, and chose to make a threat without leaving a name.
My response to the person who made the threat:
Yeah, you should probably kiss my ass. The fact of the matter is that, if you sue for defamation, you'll lose because truth is an absolute defense to defamation (that means that someone could prove all other elements of defamation and prove substantial damages as a result of those spoken or written words, but if those words are true, then too bad). You should have thought about the consequences of narc'ing before you decided to take the easy way out.
Your attempt to scare me was pathetic, at best -- I don't scare easily. If you do file some frivolous lawsuit against Expose Narcs, then we will sue you for abuse of the process, malicious prosecution, and defamation (since accusing us of falsely accusing someone of being a narc is defamatory to us). Bring it on!