5. Hank Voight
Another controversial pick, but c'mon: Hank Voight (Jason Beghe) tried to coerce Casey into letting his son off the hook for a DUI, and when Casey refused, the police officer threatened both him and his fiance, Gabriela Dawson (Monica Raymund).
The kicker? Voight actually tried to follow through on his threat, and tried to KILL Casey in the season episode 1 "Rear View Mirror." Huge party foul, to put it mildly.
Voight has come a long way since his auspicious One Chicago debut, and he served his time for trying to bump off a respected member of the city's fire department, but if you were to watch his Chicago Fire episodes is a vacuum, you'd find him pretty detestable.