Why do elves devour their slain enemies?
The ethical stance they have on killing animals and plant life makes food obtaining a complicated matter for the elven civilization, those that choose to forgoe elven ethics and dedicate themselves to the butchering, cooking and growing of plants and animals for the intent of farming them for food (and thus abusing the ecosystem) are held in lower regards in elven society, being considered "the scum and bottomfeeders" of elvenkind.
The higher castes and noble classes, who refuse to consume farmed or killed food and animals and hold elven ethics in high regard, are forced to sustain themselves on self dead animals and plants, which exist in limited supplies leading to the starvation and unhappiness of the elven high caste.
A solution for this was thought of in early elven history, what if, elves were to consume those who abuse the earth for their own personal gain?
Not only would they rid the world of evil such as dwarves and humans but they would also survive on their evil flesh!
To this day the elven warrior castes hunt the enemies of nature for the purpouse of bringing back slain intelligent beings for consumption by their leaders and generals, for the good of nature and the survival of elvenkind.
And that is the story of how napalm was invented.