I think Masuka should also turn out to be a serial killer. His vibes are twice as weird as Dexter's, and, more importantly, I think Dexter's reaction to realising he's not the only serial killer in the department would be very funny.

I'd love that fanfic

Maybe everyone in the police department is a serial killer, with the exception of Debra, the one person who actually is as straightforward as she appears.

Hah! Or Deb *is* a serial killer, as well, and she and Dexter have been hiding it from each other both without realising the other person is, but they both are really judgey and sibling-y abotu each other's kills.