He thinks he's so badass. Which he isn't.
He tends to talk to himself, and monologues about things going on in the real world. Odd, really, since he seems to say them at the exact same time as they are happening.
He disintergrates anyone who dares defy him, which is unfortunate because he is the leader of the Time Lords. Rassilon hates both the Doctor and the Master, to the point where he just tries to kill them both. He devises a plan to bring Gallifrey, the home planet of the Time Lords, to Earth, thereby rescuing it from the Time War.
He does this by implanting a drum beat in the Masters head when he was a child, (Time Travel alert here) and then using that signal to pull Gallifrey across Time and Space via a small Warp Star incased in a carbonised shell, i.e. a magic diamond.
The Master is pretty annoyed when he finds out about this, since the drum beat is what caused his insanity. So he uses his newfound powers that he gained from his resurrection to continuously blast Rassilon with electricity. The Doctor took advantage of this diversion and destroyed the magic diamond, sending Gallifrey, the Time Lords, Rassilon and the Master back into the Time War.
Rassilon was angry. He went on a rage, smashing things, shouting, and hardly noticed the Dalek Fleet arrive at Gallifrey. The Doctor from that time period used the Eye of Harmony to Exterminate everything within the radius of Gallifrey, destroying the Daleks and the Time Lords. Rassilon burnt with them, and as he did he monologued about his defeat.