We now live in a dangerous world that going to the mall or a store can be dangerous. Unstable people carrying weapons are a concern. You need to be aware of your surroundings and know where your kids are. It's sad that you have to warn your young children that there are bad people out there that can hurt them.
Always remind your kids to return to you if you are separated from them and they see a person acting strange. As a parent you should always be aware of any escape routes for your family in case of an emergency.
We teach our kids to not talk or go with strangers and now we need to tell them there are bad people that could kill them. Use your judgement on delivering that message to your kids you do not want to scare them.