So, in the scene where the doctor is transforming into the Hulk, with the Black Widow trying to calm him down, he's all angry and then basically chases her around the facility, busting up everything, throwing people around, etc.... and he's basically trying to kill everyone. But later in the movie when he transforms during that alien invasion, he's suddenly all helpful and understanding, only going after the aliens, and just hangin out with the avengers in the end like they're all buddies.