Do you mean the intentional infliction of pain that is outside the rules?
Or do you mean the high level of physicality as the inevitable result of collisions between large athletes?
Or....something else?
Because I don't classify the NFL as a violent sport. Boxing is violent; the goal is to beat your opponent into unconsciousness. Violence is a word of malevolence even criminality. Violence is gang members shooting one another. A boyfriend beating his girlfriend.
I don't find what goes on in the NFL as malevolent. There are rules against unnecessary roughness, cut blocks, late hits, hitting a defenseless receiver, helmet to helmet contact, etc. Players get suspended and fined for being physical outside the rules.
Yes, players get hurt. So? Mountain climbers get hurt. Roofers get hurt. People who play soccer get hurt.
But if players are to have the right to protest during their jobs, they also have to right to choose to be players who assume a known risk of getting hurt.....while getting paid big bucks for it. And it is hardly violence.