Although many people criticize violent movies, if there is a movie which does not contain sex and violence, who will go to see that movie? Most major movies, such as "Terminator" and "Independence Day", were really popular in spite of the fact that the "good guy" killed more people than the "bad guy" did.
In my opinion, 
Children might imitate behavior they see in the movies. I agree that movies and other media influence our lives, especially for kids; but not all children imitate violence. Most kids hate that and don't want to see it.
The question whether movie violence should be regulated or not is a difficult and complicated matter. Although I think it would be better regulated to avoid the violence more than they should.
The Violence in movies might make some kids aggressive, but they can learn about the world from that. If there is no violence in the media, everyone would have to learn only from real experience.
Many parents tend to blame media violence, but parents should teach their children that there are tons of good things and bad things, and they should help them to know what is wrong. I think government regulation is the last solution. Before that, parents can help their offspring to behave in a beautiful way.