What would you have if you didn't have a murder in a mystery? You'd have something for the lower-level readers. When you get up into the upper levels, there's nothing that will engage you or compel you or get your emotions churning like a murder. It's the ultimate stakes.
One of the biggest challenges of writing for middle-grade or even young-adult readers is that I don't want to have too much violence in it - which really limits what you can do. It's important that they're not just bloodbaths or glorifying violence. I always try to show that a person who dies leaves a hole. There's grief in my books.