Really wanna see an angry witcher? Do you? – Conversation overheard on the streets of Novigrad
The imp pestering the merchants of Novigrad turned out to be a doppler. These monsters usually avoid confrontation - to everyone's great relief, for when pinned they become extremely dangerous. Fighting them quickly turns into a fight against oneself. Or, to be precise, against a mirror image gifted with one's own strengths and abilities. In a conflict with such even odds, only luck - or destiny - decides the victor.
If Geralt kills him:
Geralt has an extra helping of both, and so won his fight against his doppler doppleganger - and, after a moment of hesitation, killed him. True, the monster had noble motives for his deeds, but that did not make him any less dangerous.
If Geralt lets him go:
Geralt has an extra helping of both, and so won his fight against his doppler doppleganger - then decided to let him go free, for unlike other monsters, this creature's motives for harming humans were noble.
In The Last Wish, Geralt talked to residents in Upper Posada about finding work for a witcher. One villager declared they had imps, which tangled horse manes in the night. However, this and many of the other so called troublesome creatures were just folklore and Geralt summarily left the area to find legitimate cases.
