Warning: this may be vaguely gruesome to people who prefer more “clean”, “heroic”, or just plaing child-friendly games. It isn’t really that extreme, though.
What do you do when you’ve killed a magic beast? Steal its organs of course! They are often either valuable, usable, or in the worst case, food! (probably… hopefully)
But what about a wizard? They can be as magical many a strange beast, if not more. What do you do when you have killed the Black Sorcerer of the Moaning Swamp? Butcher him and sell his organs of course! The Fire Wizard of Burning Destruction would pay a handsome reward for such useful bits of meat.