By far the most invidious influence is the entertainment industry. Read Marketing Violent Entertainment To Children. Then monitor what games your children play, what videos they watch and what music they listen to. Teach your children about violence and its causes yourself. Let your values, not the entertainment industry's values, dominate your child's thinking.
While you are at it, check out the Big Three. Whether you are a parent, teacher or administrator, you need to deal with these three serious issues. Good luck!