Continuity mistake: In the scene where Ethan Hawke is talking through the safety glass in the barricade to the polite leader of the gang outside, the leader shoots the loud-mouth guy standing to the right of him, using the pistol in his left hand across his chest. Immediately after that they show a top shot of him dropping his right hand holding the gun. (00:46:00)
Continuity mistake: Near the end of the movie as the family is about to be killed by the neighbors Dwayne comes in and shoots Mr Cali, then grabs Grace around the neck. You can plainly see her hands as they hold Dwayne's arm and they are empty, then just before he lets her go a knife suddenly appears in Grace's hand. The knife then vanishes when he lets her go. (01:14:55)
Answer: These purgers are pretty crazy. They were hunting this person, a homeless man, and they wanted to kill him specifically. They wanted their prize, the trophy of their hunt. And they won't let anyone stop them as it's their right. So being blocked from their target only increases their desire.
lionhead