Rosencrantz and Guildenstern Are Dead
There are other characters that seem like the bad guy from time to time. For instance, Hamlet when he alters the letter or the Player when he has his troupe surround Ros and Guil. But Claudius is bad. Real bad. He's a minor character here, but all of the bad stuff that happens emanates from him. He's like the Death Star in the play, and when you get too close to him, you're done for. We'll just list the obvious ones: killed his brother, married his brother's widowed wife, usurped the throne, and sent his nephew (Hamlet) to England to be condemned to death. Not a lot of redeeming qualities.