If your position is that all convincing self-accounts by psychopaths are themselves only convincing because the psychopath is somehow 'tricking' you, then you can take that statement and s/psychopath/any group you care to name/ and you'd never know the difference.
Don't believe me? Try substituting various bogeymen of the twentieth century: crack dealers, pedophiles, etc. You see what I mean? It always works: You shouldn't trust what pedophiles say because, well, they're pedophiles!
Generalizing, there's always a reason why someone's supposed perfidy makes them an untrustworthy speaker, and that's a problem.
One floor down, what's going on is 'begging the question' -- not in the colloquial motivates-the-question sense, but in the sense that you are assuming what you set out to prove (circular reasoning).
Watch:
CLAIM = C is a psychopath; therefore, don't trust him when he says C is good.
CLAIM' = X is a Y; therefore, don't believe X's claim that Z.
But there's still an implicit claim hiding! In fact, it's the value of Z. Let's suss it out:
CLAIM'' = X is a Y; Ys are untrustworthy; therefore, don't believe X when X says that Ys are trustworthy.
There you have it: a textbook case of circular reasoning. That's the sort of thing C is talking about: people get so emotional when they think about psychopaths, they fail to hold themselves up to the same standards for rational discourse at which a psychopath ironically excels.