He's an evil, sadistic and ungrateful jerk! I mean, he kills anyone who objects to his orders, wanted to destroy time itself, made the Master crazy because of the drums, and worst of all, even AFTER the Doctor SAVED Gallifrey, he WILLINGLY trapped him in his confession dial and wouldn't let him out even after 4.5 BILLION years passed! And THEN, when the Doctor FINALLY got out, he tried to have him executed! I mean, SERIOUSLY? Freaking SERIOUSLY?! What's his DEAL?!?
And this is why out of all the Doctor Who villains in both classic and new who, he's the third evilest overall!