Sorry that happened; Just thinking of such a situation like that makes me spooked. If one of the kids I babysit, one that I've babysat since their birth and that I consider essentially my own child, I don't know what I'd do.
I know whatever I'd do it would be extreme, and probably inadvisable though.

When people do bad things they're normally especially good at hiding it, in reality just like with a school shooter, there's almost never any real signs of an issue before they do something, just as there isn't before someone turns to drugs or violence.