Ah, Canibalism is underused in DnD. It's one great evil practice and an effecient way to prevent evil people from getting resurrected.
Which leads to argument #1:
Sometimes the most moral forces of a society will eat a great evil to purify the community affected by it. In DnD this has a very practical value, as, and I could be wrong about this, bringing back a canabalized evil is probably harder for his supporters than bringing him back if his corpse is sitting in a particular tomb. A kind of reverse version of this in myth is Cronus eating his children.
If you eat them they can't threaten you.
Argument #2:
Cannibalism is often associated with ritualistic pollution. In Greek myth it is often used as a way to defy the gods and demonstrate one's absolute power, alla Atreus and Tantalus. In other cultures the great men are required to pollute themselves before being trusted with power so that the society has a reason to get rid of them if they go mad. Welsh kings had to mate with a horse and then eat it, same principal.
Or the ritual pollution one acquires through eating a victim can serve to deter someone from killing casually.
Argument #3:
If you're going to be killing them anyways, then why not eat them? Not simply a waste issue, but also a question of doing honor to your enemies and yourself. Some cultures might see it as a way of avoiding being haunted since ghosts of cows and other things you eat don't show up to haunt you. Others might see eating something as the only real justifiable reason for killing something. So you would have to be prepared to eat a criminal in order to kill him.
I mean in the natural order things pretty much only kill other things in order to eat them. Souldn't we all obey the natural order?
Argument #4:
By being turned into meat the victim provides restitution to the killer for the pain and effort the victim cost the killer.
Argument #5:
Eating people terrifies your enemies and prevents you from having to keep captives. It is an efficient defense method. This may sound like silly logic, but it is the current theory as to why there appears to have been systematic cannibalization among certain medieval communities in the four corners region of the US.