Hactar

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.