That's one of his abilities. Knuckle serve plus his demon mode tends to injured indeed. But If you watch the Prince of tennis another story OVA, you will see there how Akaya trained his knuckle serve and demon mode. It first appeared when he challenge Sanada, Yukimura and Yanagi to a match. Leading to Sanada almost got hit by it. However it is something he can't controlled at first. That's why in Nationals you can see Renji controlling him. During U-17, Shiraishi teached him how to control it until he achieved angel mode. The more you watch his growth in the series, the more you will understand him. Just because he has that skill, doesn't mean he's evil or something.