Across the Marvel Multiverse, Ultron is a constant as one of the Avengers' main villains, with the robotic foe being most well-known for his titular ages in both the comics and the Marvel Cinematic Universe. Most of his incarnations hue pretty closely to his introduction in the late Silver Age, where he was an unfeeling, genocidal robotic overlord.
The Ultimate Universe, however, had a much different take on Ultron. Initially part of a benevolent peace-keeping force, the robot that would become known as Ultron later took on the form of his creator. In the process, he waged war with humanity over a lost love, giving himself a somewhat tragic backstory in a relatively controversial tale.
Ultimate Ultron
The Ultron robots began as a series of fighting drones created by Hank Pym, introduced in Mark Millar and Bryan Hitch's The Ultimates 2. Helping to defend New York against the invading Liberators, the war drones would later serve the Ultimates as butlers. Their appearance was also much different than their mainstream counterpart, having a more benevolent and scrawny design.
One of the robots, however, developed sentience and a personality of its own, setting it apart from the mere mindless drones that were its brethren. This Ultron also developed feelings for Scarlet Witch, which proved to be incredibly problematic given her then-current incestuous romance with her brother Quicksilver. Ultron's actions afterward would then have a dramatic effect not only on the Lensherr twins, but on the Ultimates and the Ultimate Universe as a whole.
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Ultimates 3
In Jeph Loeb and Joe Madureira's The Ultimates 3, to enact its revenge for its lack of success in the department of love, Ultron struck at the Ultimates, starting with Pym and Scarlet Witch. Pym was drugged by Ultron, allowing the robot to leak an intimate recording between Iron Man and Black Widow to the public. This PR disaster kept prying eyes away from Ultron's murder of Scarlet Witch. Magneto takes his daughter's corpse to the Savage Land, leading to a confrontation between the Brotherhood of Mutants and the Ultimates.
During the interim, Ultron, in homage to his "parents" Giant-Man and Wasp, takes on the identity of Yellowjacket, even sporting a form resembling the Marvel Universe identity of the same name. He also produces an army of robotic duplicates of the Ultimates, who, along with Ultron himself, are eventually defeated by the Ultimates. Ultimates 3 was heavily criticized and seen as a massive step down in quality from its predecessors. Characterizations were the most panned element, with many noting that writer Jeph Loeb's seeming unfamiliarity with the Ultimate Universe caused the characters in the story to not resemble their previous characterizations at all. This was matched by the character designs, which now more closely looked like the mainstream 616 universe for some reason.
As controversial as the story was, Ultron's role in it meant that he was instrumental in an even more scandalous that forever rocked the foundations of the Ultimate Universe. The deaths of both Scarlet Witch (caused by Ultron himself) and later Quicksilver in The Ultimates 3 led to the events of Ultimatum. This story saw Magneto waging war with the Earth itself, manipulating its magnetic field and axis in a grief-stricken attempt to destroy the planet. This forced Nick Fury to reveal the truth behind the origin of mutants in the Ultimate Universe, making things even harder than before for the fledgling species. The Ultimatum Wave also killed a litany of heroes and villains, with the Ultimate Universe's roster never being the same again.
Even though his actions were revealed to be caused by Doctor Doom's manipulation (a characterization that was again more in line with the 616 Universe), Ultron's killing of Scarlet Witch meant that he was the cause of the death of the Ultimate Universe as many had come to know it. Thus, though he may have appeared far less frequently than his mainstream counterpart, Ultimate Ultron's Yellowjacket sting would have a far greater reach and impact on his world.