The doppelgänger
Remember Freaky Friday? That's what a doppelgänger is; a person from a foreign existence who bears an exact resemblance to someone else. When Hermann finds a person who looks just like him, he is struck by the irony and confronts the stranger.
The affair
Hermann is emphatic. His wife loves him very much, and they're happy together. Never mind the fact that his wife is indulging in a full-blown affair, right under Hermann's nose.
The incest
What's worst than a wife cheating on a husband? A wife cheating on her husband with her own cousin. The addition of incest into the story underscores the accidental themes of embarrassment and humiliation.
The murder
The novel seems to be heading toward a Freaky Friday story where both characters switch lives. The reader might even suspect that the novel will be about the rich man learning about poverty and suffering, but instead the rich man just murders the homeless man. That's not exactly The Christmas Story. It comes as a surprise to the reader, but it certainly comes as a surprise to Felix himself.
The doppelgänger, redux
The whole point of the plot twist at the end is that actually, really, Felix did not look like Hermann. Hermann's psychosis was making him project himself on others. Perhaps it's a statement of self-hatred, to need to kill someone who looks just like one's self.
