Avengers: Age of Ultron (May 1, 2015)

The second of the Avengers, Age of Ultron sees the rise of a powerful and nigh unstoppable AI.

Searching for the means to protect the world from any threat, Stark creates Ultron, a psychotic AI who sees humanity as the only threat to Earth – and so he seeks to destroy it. Besides setting up quite a few movies, Age of Ultron shows how Iron Man makes up for this mistake – by creating another AI, Vision, who seems to be even more powerful than Ultron and who, ultimately, defeats the latter.

Bruce Banner also comes to terms with who he is, but in an unexpected way. After threating an entire city, he realizes that the Hulk cannot be contained – so, in order to make amends with who he is, he leaves the team (and, as it was already confirmed, Earth).

Scarlet Witch and Quicksilver are the ones who turn against their former ally and support the Avengers against the mad AI – their stories begin with their hate against Tony Stark and the other heroes and end up with actually entering the team. Quicksilver goes so far that he even sacrifices himself in order to save Hawkeye and a child.