It is interesting that the avengers could not stop Ultron, it was the introduction of scarlet witch and vision that stopped Ultron, not the avengers. If we look at the first half of the movie, Ultron really does kick the avengers ass with help of course. But Ultron deceiving the twins getting them on his side, seeing the avengers scattered going into hiding on hawkeye's ranch, the plan has gone so well for Ultron he has become a one man show. Ultron he is transferring his life force into vision, at that point of the game Ultron has won, it was that 3 minutes that changed the game back to the avengers favor. Because before that, Ultron has the vision body, he has the vibranium, he is transferring his lifeforce into the vision body, the twins are very much on his side, he has blitzed the avengers within 1 hour of the film, he becomes vision the game is up for the avengers. Everything has gone to plan for Ultron, but Ultron's rage becomes his weakness and the plan unravels for him
Yea Ultron was a character that was complex and his hatred was very interesting which was his downfall, he could not quiet play the poker game to keep his cards close to his chest, and keep scarlet in the dark. Ultron's weakness there is so much rage in Ultron he could not keep it in check, she feels his rage when she looks into his mind. I thought he was by far the most interesting character in AOU, and every time he was on screen you are fixed on Ultron. He has a vast knowledge of the history of earth, every book that is out there he knows, his biblical personality from gaining the vast knowledge of the testaments, he uses has a metaphor of hatred and bringing human race to an end. We see how dark and totally out of his mind Ultron truly is, when he responds to quicksilvers question when Ultron says (the human race will have every opportunity to improve) and if they cannot? Ultron responds, ask Noah.