If you are close with your parents, tell them you stumbled across something violent online that upset you and was disturbing. Tell them you try to be as safe as possible online but somethings stuff pops up unexpectedly and you reported it but still saw some of it and it was really sick. Hopefully they will have a chat with you and calm you down.

Parents biggest worry is usually sexual stuff not violence, so I think you will be ok.

Don’t worry too much mate, the world can be a dangerous place but most people never see any of that IRL. I imagine you’re a little sad for the poor people who died those tragic deaths too, all of them were also someone’s child.