How deadly is he? He singlehandedly slaughtered the inhabitants of the fictional European nation of Slorernia, before enslaving the alien robot race the Phalanx and taking over countless planets as a result, for his own nefarious ends.
Who's responsible? Hank Pym (the superhero known as Ant Man, Giant Man, Goliath, Yellowjacket and, currently, the Wasp) built the original Ultron as a lab experiment in AI, but it was a little too successful; rebelling against Pym, Ultron has been responsible for all of his subsequent rebuilds and remodels.
Last seen... apparently being destroyed by Quasar and Adam Warlock at the end of 2008's Annihilation: Conquest crossover. But that never lasts.