The reason for their being slain is surely not to feed the birds (that’s the effect; not the cause). The cause for their death is found in Genesis 2:17 (Romans 6:23): rebellion against God attracts his ultimate penalty. By aligning themselves with the beasts, the kings and their armies commit rebellion against the King of kings.
21 And the rest were slain by the sword that came from the mouth of him who was sitting on the horse, and all the birds were gorged with their flesh.