Season: 3 (12 episodes)
Killer bio: Self-righteous Assistant District Attorney Prado breaks the law to put murderers behind bars. Ironically, after he becomes friends with Dexter, who puts him on to murdering criminals, he kills an innocent rival defense attorney who's trying to prove that he's dirty and plots the murder of the police lieutenant who's investigating him to keep himself out of jail. Objection! Double standard!
What he does to Dexter: Prado manipulates him into believing he's trustworthy by presenting Dex with bloody evidence of his involvement in a murder (Dexter later discovers it's cow blood). That's no foundation for a friendship! And neither is freeing another serial killer, The Skinner, so he'll eliminate Dexter.