Daycare would be a far more significant factor than a midnight movie. Kids are not getting enough time with their parents for their parents to help form their morals, so they get their morals elsewhere.
Kids used to play cops and robbers. Violent games are as old as mankind. War games, with simulated torture, are also as old as mankind.
A movie is not going to cause people to go out and commit mass murder.
Can give kids nightmares for years.