Redditors think that muggles know the word 'Abracadbra' coz wizards were using the killing curse on muggles. EEEEEEK! They reckon that after the Statue of Secrecy was passed to hide magic from non-wizards, that the term got twisted and changed until it was used in fairy tales and stories... and everyone basically FORGOT that it was originally used to... ya know, murder hundreds of people.
But wait... there's another twist that makes this worse! Redditor Canvaverbalist thinks that maybe actually 'Avada Kedavra' was originally intended as a spell to heal...
"Because this spell was at first created to cure, what it did in fact was killing the bacteria and viruses in the body, thus healing most illness. But then some clever magician discovered they could expand the spell, make it stronger and even more deadly. That's about when the magicians started to be seen as a treat in general, frowned upon and excluded from society. This is why this word/spell is such a forbidden one, not only because of its direct consequence but also because of all the history it wears."