As the Avengers make their first move on Ultron, he uses Scarlet Witch to give the entire team (except Hawkeye) visions of their worst nightmares.
Thor saw that he destroyed Asgard, Tony killed the Avengers, Natasha went back to the red room and Captain America was alone after the war. What did the Hulk see during his nightmare vision?
Banner's worst nightmare is killing innocent people, he has a major fear of losing control while in a populated area. This point is made multiple times during the movie ("don't turn green") and reiterated at the end of the movie:
Romanoff: So what's our play?
Banner: I'm here to get you to safety
Romanoff: What about the drop-zone evacuation?
Banner: We can help with the evacuation but I can't be in a fight near civilians and you've done plenty...
The Hulk lived his nightmare vision. Banner turned into the Hulk and ran into a heavily populated city full of civilians and it took Stark in the Hulkbuster armor to stop him.
He saw what he feared the most. Losing control of himself and harming others. Ironically, this is exactly what he did. Just shows how sometimes, fearing something will make it a reality.
