Hactar, from Life, The Universe, And Everything, is an ancient, sentient computer that was ordered to design the "Ultimate Weapon", and pulverized for disobeying that command (it couldn't conceive of any possible scenario where destroying the universe would be a preferable option). Set adrift as an interstellar cloud of still barely-functioning dust, Hactar spends the next several billion years manipulating another planet's inhabitants into reaching the same xenophobic cultural state as its creators, and then reinventing and triggering the same superweapon in order to put an end to all of existence. Hactar explains its motives as mostly simply fulfilling its original function, but partly to take revenge on the Universe for the eons of suffering it has endured as a result of its original decision.
Hactar was very powerful, even in his pulverized state as atoms he had still power enough to make objects. He was the one who smashed the ship into planet Krikkit in order to make the Krikkit people warmongering and xenophobic. He had fun manipulating races and making them aggressive.
Hactar saw his interrogation as a joke and manipulated the environment to make his surroundings that of a psychiatrist's office, and made himself sit on a five foot couch, which was remarkable because he himself was a thousand miles long.