The Marvel Comics Web site describes Ultron as "a criminally insane rogue sentient robot dedicated to conquest and the extermination of humanity." To which you might respond, "Well, OK, that sounds pretty negative, but surely he must have a few good points too." Not to disappoint you, but he doesn't. Ultron isn't a robotic slave driven into a rage by his servitude, like Rossum's robots, or an avenging angel like Gort, who seeks to deter humans from their own evil nature. Nope, Ultron is just a thoroughly unredeemable mess of metal, who just hates people because, well, that's what it does.
He gets that from his human creator, the scientific whiz Dr. Henry Pym, who initially gave Ultron a pathetically weird body -- basically, a torso on tank treads with spindly arms -- and endowed it with a copy of his own twisted brain patterns as operating software. As a result, Ultron quickly developed an intense hatred for both Pym and the human species in general, and after overpowering his creator and taking over his lab, the machine rebuilt himself into a broad-chested behemoth. Pretty soon, he's tangling with the Avengers, and creating a series of new and even more powerful bodies for himself. In the process, Ultron expands his mission, aiming not just to wipe out humanity, but all organic life as well.
But even exterminating robots get lonely. At one point, Ultron tried to create a mate for himself called Alkhema (aka "War Toy"). But first romances usually end badly, and this one was no exception; after quarreling with Ultron about how quickly all life on Earth should be wiped out, Alkhema not only stomped off in a hissy fit, but actually helped the Avengers foil one of her ex's fiendish plots. That'll serve him right [source: Marvel].