Dexter (2006)

Dexter Morgan is a Miami-based blood spatter expert who doesn't just solve murders; he commits them too. In fact, he's a serial killer -- but he only murders the guilty, so he feels justified with his lifestyle choices. His policewoman sister and his cop co-workers have no idea Dexter lives a double life; however, adoptive father Harry knows his secret, and does, in fact, help Dexter hone his "skills." It's a unique brand of justice for which charming Dexter feels a psychological hunger.