So you know how sometimes grown-ups get really, really upset and do things they shouldn't do? Like when you throw a tantrum because you didn't get a toy you wanted, but their bad thing hurts other people or even themselves.
Murder-suicide is a really, really bad thing that a grown-up can do when they're really, really upset. It means they hurt someone else (that's the murder part) and then hurt themselves too (that's the suicide part).
It's like taking away someone else's toy and then breaking your own toys because you're so angry. But instead of toys, it's people's lives, which is why it's such a terrible thing to do.
It's never okay to hurt someone else, no matter how upset you are. If you ever feel like you want to hurt someone or yourself, it's important to tell a grown-up you trust right away so they can help you feel better.