My kids are still pretty little (oldest 7) so they can still be shielded from most of the tragedy out there. I think it's important to be honest and answer their questions. However, not to dwell on the bad. Yes there are bad people out there and they do bad things. You can't stop living your life because of it. I also love the quote out there from Mr. Rogers about looking for the helpers. I think it's good to focus with them on those people that are "fighting the bad guys" and helping the victims. There are always more good people out there than bad.
"When I was a boy and I would see scary things in the news, my mother would say to me, "Look for the helpers. You will always find people who are helping." Fred Rogers