Agreed. Even without promising Gentile a deal, the fact that they needed to groom him on how to lie to their own auditors (seemingly about entrapment, as well as the various breaks with protocol) speaks to a failure on their end first.

"In two years they’d spent a few hundred thousand dollars on expenses, Gentile says, including about $15,000 for his travel. The agents told him that an investigator would be asking him some questions, and they wanted to go over what to say... They advised him to say he hadn’t directed any criminal activity. (“Of course I did,” Gentile says.) And they said not to mention his gun."

That's not an account of a properly-handled operation with an unreliable informant. Gentile appears to be a bit round the bend, but that's not the only thing wrong here.