Or violence acts in general?

I was talking to my mother about this earlier this evening, and she just said that "bad people do bad things to good people" but that really felt too simple.

I know there are a whole lot of reasons I could probably think of, and it's definitely a conversation that I won't have to have for a few years, but how can you explain something like 9/11, or Columbine, or Newtown, or Aurora, or the Marathon today to a child? In a lot of those cases I can tell my child they were sick in the head, but is that something that a child can understand and is it simplifying the issues too much?

How are you planning on handling these conversations? If you have older children, how do you explain things like this to your children? Am I over-thinking it?