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.