2. Sean O'Neal
Sean O'Neal (Jefferson White) is the most recent character to appear on the list, as he was a recurring presence throughout season 10. He was clearly a bad person from the jump, and he was made more despicable from an IU perspective because his dad, Patrick O'Neal (Michael Gaston), was a CPD Chief.
Patrick ensured that Sean could not be intimidated or investigated seriously by the IU. When Sean's human trafficking crimes got too gruesome and too frequent to defend, Patrick tried to kill his own son. The killer lived through his gunshot wounds, however, and is currently behind bars.
Related: What happened to Adam Ruzek's dad?
Creepy presense, through and through.