If you say that good people suffer because of evil, that projects the possibility “outward” into the real world, where it could actual harm (and certainly cause anxiety) to a child. It’s true that evil is part of the world, but there’s no need to allow fiction to threaten a child’s security. (Even a limited exposure to history or current events is enough for that, but that’s a different thread.)
If you emphasize that the author has made everything up, a child can still emotionally engage with the characters, while recognizing that they haven’t “died,” because they were never anything but words on the page and images in the imagination in the first place!
What I found to be the REAL tragedy of this story was - that at 38 - Severus Snape died a virgin.