Explaining violence to children is touchy, obviously. It is something that may be harder than explaining why they saw daddy in a Santa suit last Christmas. But it all depends on your level or preference of honesty on the subject. As a parent, how much do you want your child or children to know? There is a level of innocence you want them to retain, but on the other hand you want them to also be a bit knowledgeable on the subject.
I would start by saying a vague response, such as "Well, some people are just mean honey. Some people that live in this world are the nicest people you could ever know, and then there are those you have to watch out for. We cannot explain why they are the way they are, but all that we can know is that they are different from others in a way that they like to hurt and don't mind making people cry. They are the grown up bullies."
And as your children grow, extend from that in more specifics.