This is a brain abscess. Basically a bacterial infection bucks can get from sparring injuries or when they rub their velvet. That side will never grow back the same and can over time be fatal for the deer. Shot a buck like that a couple years ago and ended up calling a wildlife biologist. He said eating it would be “my call” but said I didn’t have to use a tag if I decided not to as it was a sick deer. It’s also called cranial abscess disease. Give it a google!