During the course of the Avengers, you see Mr Banner talk about his alter ego as just another entity inside of himself. Afraid, reluctant and very much un-willing to let him out, even if it means his escape from being captured by S.H.I.E.L.D. Citing Hulk's violence, lack of self-control, and raw power that would come of unleashing such a creature.
However, during the argument on the plane as well as for the final battle, Banner is willing, and eventually does let the beast lose, seemingly without knowledge as to whether or not he can control him properly.
Why does Banner choose to release the Hulk and where does he get the confidence he needs to be sure that he can point the indestructible beast towards the enemy, and not towards the Avengers?