Your logic chop fails because one of the elements of psychopathy is being a convincing liar.
"Does my bum look big in this"? "Hi, wow you look great...."
And so on.
Humans lie. If they didn't, they'd be in constant conflict.
Reading the top parent of this thread was very interesting because he was thinking in exactly the way I had been conditioned to think at that time in my life.
When a person in your life lies to you or manipulates you, you will certainly begin to question the value of other interactions from that person. Once you have confirmed that the person has a personality type that makes them particularly likely to lie or manipulate (compulsive liars, psychopaths, criminals, what have you), you must be careful about your interpretation of any communication. But here's the real difficulty: If a psychopath is trying to manipulate you and knows that you are conscious of it, the manipulations morph. They adapt to your particular defense. In the institutional setting, they're in it for the long haul so they don't mind missing a couple times. In fact, getting caught is often part of the manipulation.
I wandered off my initial topic here so I better wrap it up.
Certainly lots of paedophiles are terrible liars.
Thus you can't debunk him just by saying "Don't believe him because he is a psychopath is insufficient".
If you want to pin all of his logic on that baseline theory (which would be stretching it), then your replacement no longer works, as crack dealers aren't known for being persuasive and manipulative in words in that way. Similar for most other distrusted groups.
But we are both just conjecturing :).
He made no such generalization. He simply talked about this one self account.
See, I positioned myself for not getting it quite right -- I can say, "this means I'm probably less psychopathic than you."
Monster! :D
Once you see that, the material of the letter takes on a different aspect. Even if it weren't from a psychopath, it would be enlightening.
(I have other issues with your use of naive logic based on non-probabilistic propositions, but you may be doing that to dumb it down for the audience.)
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.