Brutality to one's opponent and damage to their own body are not mutually bound. I think the psychology of aggression is what causes the damage to the practitioner, but being brutal does not require aggression. A true professional who immerses him or herself into violent situations in order to make a living should reach a point where they do only what's necessary to get the job done and go home. Joint, nerve, and brain damage arise from improper practices. Why hit the skull with one's knuckles, damaging them, when the neck is only a few inches away? Why become overexcited and adrenally charged when the situation requires calm and decisive action? Why spar with a person who wants to kill you? Martial arts that enforce mandatory aggression (Krav Maga for example) in order to be effective are just as terrible as the ineffective arts that make non-aggression mandatory. One damages your body and psyche through training, while the other leaves your body and psyche unprepared for high-stress encounters.