Revealing mistake: When George Clooney is talking to Juliette Lewis towards the end of the film, it is painfully obvious that his tattoo has been smudged. (01:40:35)
Revealing mistake: The closing shot is obviously on a matte painting - the perspective on the ziggurat's corners is off. (01:42:15)
Revealing mistake: Shortly after passing the border Seth is talking to Jacob who's supposed to be driving. But the reflection in his glasses shows that they don't move.
Revealing mistake: When the gas station clerk jumps up from the floor, you can see the protective clothes he's wearing to protect him from the fire.
Revealing mistake: In the scene where Kate is aiming her gun at Scott while he is being seized by several vampires, it is clearly a dummy surrounded by other dummies when the shot is made and the group explodes.
Answer: Because he is a sadistic lunatic and he wanted to. He has a lot wrong with his brain, paranoia, psychopathy, hallucinations, sexual predation. Probably more. He is a deeply disturbed individual and killing Gloria gave him pleasure.
lionhead