You say you don't necessarily mean "con", but "manipulated" definitely implies you feel you were led to believe something likely false that you wouldn't have otherwise via trickery, etc.
The letter may have no long-term purpose, but it might be written that way because the author is so used to this way of thinking.
Despite his intent, I believe that I left the letter thinking exactly what it was designed to make me think. Therefore, the actual mechanics of the letter (which were outlined well by tomstokes, in my opinion) were excellent at eliciting what I assume is the intended response. This is akin to successful manipulation, but may not necessarily be from a place of malice on the part of C. However, C's changing my opinion of psychopathy does in fact place him in a more advantageous position if the two of us were ever to meet, thus I was manipulated.