Nero

You could argue that Nero was not all bad.  At the beginning of his reign, he was viewed as a good ruler who held up democratic ideals, and gave speeches saying so.  But, he did not write these words, they were written by Seneca, his tutor and caretaker.  Later was when Nero started to show what I believe were his true colors.  He had his mother killed, then exiled his first wife (later having her killed) because he wanted a prettier wife.  Then, when his second wife was pregnant with their second child, he beat her to death.  Nero was clearly mentally unstable, and a very dangerous person, not just to those close to him but also to the general populous of Rome.  It is widely believed that he set a giant fire in Rome just so he could build a new house, then blamed it on the Christians.  So, Nero was really only bearable when he had Seneca keeping him in check.  After that he was able to let loose and do all of the crazy evil stuff he really wanted to do without opposition.