Both Mc and ml are psychopaths (I think), or at least some form of sociopathy. Mc is a journalist who has a very low moral threshold : he likes to collect all the dirty, dangerous, sordid secrets of other people, and keep it to himself. He gets to interview a serial killer (ml), thanks to his ex who is ml's psychologist.
He wants to know everything about ml's thought process, and doesn't intend to tell the police. He even thought that ml should always be free, hinting that he somehow wants to see more killings I think.
ML pretends that his first victim wanted to die, though Mc knows better. To Mc, Ml is a liar who covers up his madness and acts, switching between a bright and a dark personality in a sense, with the bright one sometimes used as a facade.
Ml becomes interested in Mc as a sexual partner, though he doesn't like being called out by Mc on his lies/dark side. He however seems to become addicted to Mc. Mc resolved to seduce him with his bed skills in order to get his story.
I don't know how this will develop, or if it will end well, but it's very interesting. Both are each other's mirror, they recognize the other's darkness. I wonder if they will fall in love.
Interview with a Murderer