I don't think anyone is to blame for this tragedy except Cho. They can sit and stew and wonder 'what could we have done differently' to prevent what happened. You can't sit and say what if I did this or what if we had gotten him help when he wrote something violent in English class. You never know exactly what the person next to you is thinking. You can't see when they are going to fly off the handle and murder innocent people.
Teach your kids to respect others no matter what their income, race, sexual orientation, religion, eye color. Teach them to respect life. That's all you can do.