I'm an avid ice-hockey fan, and an occasional (bad) player. I also used to race motorcycles. I get what it is to do something that carries with it significant risk for life-changing head injuries. You do it either because you love it that much (which of course, few understand), or because it's become your livelihood.
I also grew-up with a mother who was raised in an abusive home—physical and emotional. Her father regularly beat and cheated on his wife. Her mother turned to manipulation, denial, and drinking, as her way out. That was the 1940s & 50s.
The most heroic thing I think any person can do, is to do what my mom did: successfully break the cycle of abuse. It didn't come "naturally" or easy. It required a tremendous effort on her part—years of therapy, constant neurosis as a parent (that yes, did annoy her children), an intense struggle with depression, and by no means was she a perfect parent. As my brother and I (only recently) came to learn more of our mom's childhood, the intensity of her struggle—and for decades had been—became clear to us. How much we grew to admire and love her that much more for it, is beyond measure.
It's not natural to headbutt one's partner, because they don't want to have sex with you. Nor is it natural, to beat another individual with less than half the physical body-mass or strength that you do, into unconsciousness. Especially when that individual is the one you love the most in this world. I don't know why each individual person does it—but I do know, it's rarely because they're a decidedly "bad" or "immoral" person.