1. Grant Smith
Grant Smith (Guy Burnet) is the absolute worst. He's the ex-husband of Stella Kidd (Miranda Rae Mayo) and its baffling to conceive of a time in which the two of them were on good terms. Smith was abusive towards Kidd when they were together, and even attacked her with a knife after they broke up.
Smith's actions are loathsome, but to make matters worse, he seemed to relish the trauma he inflicted on his ex-wife, and even played a role in Kidd and Severide breaking up for a brief period of time. He was bad news every second he was onscreen, and the damage he caused to some of our favorite characters goes a long way in making us hate him.