LAST EDITED ON Oct-30-20 AT 03:04 AM (EDT)
 
>I would have thought that killing a troll would remove some of the
>magic in it, and thus troll fat would be harvested from a living
>specimen, sort of a "catch and release" operation like gathering blood
>from horseshoe crabs.

The problem is that gathering blood is relatively non-invasive, and horseshoe crabs are not generally capable of ripping out a human's guts with their claws. In comparison, gathering troll fat is going to be invasive, and trolls are not only capable of disemboweling a human, they consider it to be a reasonable action.

Peter Eng
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To be fair, some trolls prefer decapitating humans to disembowelment.