Much the same as using Mafia members to act as informers. Informers and spies are not necessarily nice guys.
And, frankly, Stalin was just as bad as Hitler, although FDR called him Uncle Joe and the NY Times insisted that he was a nice guy—and won its first Pulitzer Prize for whitewashing his mass murder in Ukraine.
That’s a false comparison, since nobody uses a rival gang to spy on your primary target gang.