Continuity mistake: When Dark Hulk arms the nuclear warhead, the orientation of the timer is the same as the control buttons. When Banner disarms the warhead, the orientation of the timer is now upside down respective to the control buttons. (00:18:00 - 00:20:26)
Continuity mistake: After the alien entity has been purged from Bruce Banner's body, he is on all fours and tilts his head back, with the warhead behind him. When the shot changes, his head is pointing forward again and the warhead is now directly in front of him. (00:20:15)
Continuity mistake: When the alien entity transforms Bruce Banner's body into a purple-skinned Hulk, his hair also turns black. When the shot changes, his hair is back to being brown.
Answer: The Hulk is a giant brute who smashes everything in his path when he is angry. In Ang Lee's "Hulk," General Ross, played by Sam Elliot, explains it to his daughter.