8. Richard Beck
Richard Beck (Lee Tergesen) didn't have many redeeming qualities. He was a domestic terrorist who built bombs and forced members of his family, including his daughter and grandson, to partake in his planned attacks. It's no wonder Adam Ruzek (Patrick John Flueger) went undercover to stop him.
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We got to spend a decent amount of time with Beck in season 10, and given that he puts the lives of several PD characters in danger (his grandson shoots Ruzek in the stomach, for crying out loud), it was rewarding to see Atwater finally take him out.