The imaginary quality of literature
There is an underlying joke in the end of the novel. If the novel had been made into a movie, either Felix and Hermann would be played by the same actor, or they would be made to look different. But in the novel, it's perfectly ambiguous. We know that Hermann sees an uncanny resemblance, but Felix doesn't see it. In the end, the joke is that actually, the two men look nothing alike. The point is that the novel indicates the fact that the reader was not able to know enough about the men's appearance to know whether or not Hermann would get away with it.
Psychopathy
The book follows the life of a cold-blooded murderer. He is not a wrathful person. When for instance, he finds his (psychopathic) wife engaged in full-blown incest, he just walks away. But then he murders a random person. The effect is that the man seems to be genuinely insane, and psychopathic to boot.
Identity
By not allowing their family ties to stop them, Lydia and Ardalion engage in forbidden love. By not seeing his own face correctly, Hermann mis-identifies a doppelgänger. Hermann's act of murder is to kill someone like himself, implying that there are boundary problems in the psychologies of the characters, especially in their senses of self.
