It really does go with everything. Norman looks so peaceful and serene when he's cutting animals open, removing their organs and then stuffing them for a display so that they can live forever in a world where the living can admire the dead. Who wouldn't love a bit of death sitting on their mantlepiece? If anything, Norman is breaking down the barriers of what society believes to be a taboo occupation, or in this case, a hobby. You do what makes you happy, even if no one else really gets it and thinks that you're a bit weird. It definitely isn't just a sign for you being a serial killer.
