I have played the Witcher 3, exchanged dialogue with 5 trolls already, and decided not to kill them. All 5 trolls were involved in specific quests. I have killed two trolls of which I did not exchange dialogue. I have yet to receive a Troll Mutagen.
How does one obtain a Witcher 3 Troll Mutagen from a non-quest specific troll, at least one of which you never exchange dialogue with?