The Hulk may be the strongest of The Avengers but he's also the most unpredictable and uncontrollable, making him the most dangerous - in a bad way. We saw hints of this in The Avengers where he lost control on the S.H.I.E.L.D. Helicarrier and attacked Black Widow (Scarlett Johansson) and it's an issue that Dr. Bruce Banner (Mark Ruffalo) continually deals with in the comics.
Banner and The Hulk are both battling for control and despite Banner seemingly embracing the green beast for the climactic battle of New York at the end of The Avengers, and despite Tony Stark (Robert Downey Jr.) expressing confidence in Banner's wild side, Banner can never be fully in control. In the sequel, The Avengers: Age of Ultron, Ultron aims to break the team, using a few super-powered individuals of his own and an army of robots, and it's becoming increasingly likely that part of his plan - much like Loki's last time - might be to turn the Hulk against the other heroes and potentially, innocent people.
We saw the concept art where Iron Man's Hulkbuster armor fought the Hulk and at Comic-Con over the weekend the first Avengers: Age of Ultron teaser trailer heavily emphasized Banner/Hulk seemingly losing control and fighting Iron Man. In our chat with Ruffalo after the Marvel Studios Comic-Con panel, he promised that fans will see more Hulk in the sequel and that "Hulk Hulks out more."
"You're gonna get a Hulk that's meaner and more angry than you've ever seen."
If Hulk is more "mean" and potentially less in control than ever before, is there redemption for that version of The Hulk? Our own Kofi Outlaw asked this to him on the Marvel SDCC press line:
"Uhhh... if you ask Banner I feel like he would tell you 'no.' There's only so far that he could ever go before he feels like there's no coming back and we might have crossed that threshold this time (laughs)."
Banner not forgiving his alter ego, the "other guy," is a key point and story hint by Ruffalo...