I'm about to go into the Elusive Thief contract, fully intending to kill the poor little guy and take his mutagen to brew my last decoction. I started a NG+ on Deathmarch difficulty, and the only decoction I'm missing is the Doppler. I had found out well beyond the point of having completed the contract, and since I play Geralt as a protector of both humans and non-humans, I spared the Doppler and had no way of going back to change my decision. So, when I found out the only way to get the Doppler Mutagen was if you killed the Doppler in the Elusive Thief contract, I was so disappointed. But, I figured I could reasonably obtain all Witcher gear and concoct all potions and decoctions, so I found fulfillment in that idea. It took me a while to get over how I can't reasonably have all the swords in the game and have coin to spare for big spending, as I found myself having to sell my swords at one point (while keeping relics).

So first and foremost I'd like to say I'm a completionist. I'm just posting this here because I need to get this weight off my shoulders.

Witcher 3 doppler quest