It sounds a little like those folk lullabies of old that mothers used to sing to their children when they didn't want to fall asleep. So they tried to scare them with some mythical monsters coming to eat or kidnap them if they don't behave themselves.
Wow, thank you for your translation. I like the way the English version flows aurally, but that Polish version is more clear about the meaning, I think. I really like that too.
I also think it's a human mother who would sing that song traditionally, rather than monsters among monsters. I feel like the fact the vampire knows about it says to me that there this general, almost universal idea about witchers -- both the monsters the witchers kill and the people witchers protect are mortally terrified of them. This trailer makes me think of that scene in one of the short stories with the deoval and the elves of the mountains -- the old woman describes from a bestiary witchmen and their hideous nature. Humans wrote this bestiary and illustrated witchers in it as terrible grinning mythic beings who steal children and bring curses while they greedily take gold for the nasty work they perform. I think they're perfect candidates for 'eating children whole' in lullabies.
As for Geralt not taking the trophy -- the way he looked at her and then got on his horse almost in a hurry and his expression seemed to show to me a type of anxiety. Maybe about what he'd done, maybe the reminder of the song. I think he just wanted to get out of there. I don't think he had an issue with carrying her head on his steed, since he clearly intended to take the trophy before he passed out. That moment when he woke up, however -- the song changes from singing about the night and those who lie awake to how terrible witchers are as soon as the sun rises after the title. I think that holds some significance to Geralt not taking the trophy and the way he behaved after he'd woken up.
The woman was dead -- she couldn't be singing the song, maybe it was a reflection in his imagination.
//i over analyse everything ok dont take me srsly pls