We all have what Karl Jung called a shadow self. That hidden self wants to do terrible things, be violent, cunning, selfish, greedy, rapacious, etc. The way we grow up and make friends with that shadow self is by recognizing it exists and negotiating with it so that we can harness its strengths and control its impulses. Children who never learn about the darker self of humanity, who are sheltered from the past emergences of the dark side, risk being overtaken and overthrown by it. Even long ago, people understood this, and made dark fairy tales for children to get them, in a fairly secure space, prepared for that greedy violent ape that lives within. Children beyond the fairy tale phase, like 4th grade and up, can begin to learn some of the dark history mixed in with the light - its incredibly important to becoming a whole person.