Animeleague

In an alternate 21st century, rich mafia members can turn into "Demon Kings" through increasingly popular ether-enhancement surgery, and the bounty hunters called "Braves" are the ones called upon to take them down. Yashiro only wants the simple pleasures in life - pizza, beer, and card games. When three young Braves offer him a majestic sum to be their private tutor, he agrees solely for the money - but is it worth what he's getting himself into?

What a strange premise - but then we did see humans becoming monsters in One Punch Man.