2. Trenton Lamont
Trenton Lamont (Robert Knepper) was a capital C "Creep." Not only was he a serial arsonist who damaged countless areas and took countless civilian lives, but he would take people's identities and use his unassuming presence to stalk members of the 51, particularly Dawson.
Lamont also took an alarming interest in the death of Leslie Shay (Lauren German) during season 3, which was still a really sensitive subject with fans and made him feel even more sleazy.
It was, frankly, rewarding to see Lamont get his just deserts when he was shot and killed by Dawson's brother and Chicago PD star, Antonio Dawson (Jon Seda).