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)
Other mistake: When Dark Hulk lifts up the middle army tank, he rotates the tank towards the two tanks on either side of it and fires the tank gun at both of them. Since he's holding the tank from the bottom, he has no way of controlling the tank gun, and there's no reason the tank's gunner would fire on his own men.
Factual error: When the Dark Hulk is attacking the city and destroys a helicopter, the occupants can be seen with their parachutes already deployed immediately after the helicopter has exploded. Not only would they not have had enough time to already be parachuting to safety, but given that the helicopter was only a few stories high in the air, they wouldn't have had time to safely deploy their parachutes in the first place.
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.
Other mistake: Throughout the episode, you can see that the animators drew Doctor Strange's fingernails, even though he is wearing gloves.
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.