This is a no-brainer. When you leave the kind of marks that were left on Peterson's son, you have gone over the line -- way over the line. How come he doesn't understand this? How blind does a man have to be to not see that he has a problem and needs to correct what he is doing?
I spanked all my kids. My first one got the worst of it. I would give him five swats with a belt, pants on. If he had really pissed me off, the pants we off and the underwear remained on. As time went by, my wife managed to convince me that three swats were more than enough to drive a point home.
In addition, before the spanking, I would recite the child's misdeed back to him/her, have him/her acknowledge it, and then tell them why it was wrong. After the spanking, I would have the child apologize to Mom (it was 99% of the time Mom who caught crap from one of the kids) and then pray and ask God for forgiveness of the offense. Then there were hugs and promises not to do it again.
I don't see Adrian Peterson doing any of this. What I see, in the marks on his son, is a man in a rage and completely out of control.
He needs serious professional help.