I would tell a child that there have always been bad people in the world. There were bad people when their Mummy and Daddy were small (mine grew up in the shadow of the bomb) and when Grandma and Grandpa were small (we were both born during WW2). Nothing had ever happened to us or any of our friends and relatives, so nothing was likely to happen to them.
And that is exactly what DS told DGD, aged 8, when she too asked questions about last week's events. She found this re-assuring.
What I would never do is not answer DGC's question or tell a lie. I would always tell the truth, but temper what I said and how much I said, to the child's age, knowledge and temperament.