How do nursery rhymes become nursery rhymes if the words are frightening?

Nursery rhymes we know today come from European folk lore, for the most part, British (English speaking) folk lore. Their origins had a number of purposes before the average household had books or were literate. Some had hidden political messages from times that to actually speak ill of or make fun of the ruling class could have dire consequences; some commemorated historical events; some were devised to teach children a lesson or moral; some were used for their sing song rhythms to soothe a baby or to interact with a toddler to make them smile and laugh.

It's difficult for us to imagine what lives were like for the average European of hundreds of years ago; we know much of the history but not much about what they thought about or talked about as they went about their daily lives. When things were harsh for them, they were very harsh. Something like the plague is said to have been the origin of Ring Around the Rosy, describing the redness around the boils, people carrying flowers in their pockets to hold over their mouths and noses to avoid being infected, the end with so many people dying that piles of bodies had to be burned, then we all fall down having become sick ourselves. Pretty harsh stuff when it's happening all around us. Certainly more frightening than abstract tales of wicked witches or princes fighting fire breathing dragons. A rhyme like Ring Around the Rosy would have been sung over and over by children as a way to cope with the reality. It's possible that the rhymes that sound scary to us may have been seen from a different perspective to the people who created and perpetuated them.

Even more recent history gives us a good example of real life horror turned into children's fun, Lizzie Bordon with an axe...