IMO this was 99% about a joke from earlier in the show directed at the Smiths and making light of their open relationship which I think Will struggles with a lot. His pride was hurt, he felt emasculated, so when this rather innocuous joke was said he acted. The entire way he behaved after while accepting his award made clear this was about Will's feelings and "manhood".
Their open relationship is joked about endlessly in Hollywood, Will has been called a Cuck. Society doesn't accept non conventional marriage situations very readily, and it certainly seems that their relationship status being open is more Jada's demand than Will's preference.
When he accepted his award, he reminded me of a grade school kid who was being bullied who finally couldn't control himself and attacked someone, he was struggling to make excuses for his loss of control, he was emotionally overcome with feelings he didn't understand and couldn't control.
Toxic Masculinity elicits violence when someone's "manhood" is challenged, I believe that's what we saw here, Will Smith's fragile ego was hurt by the joke about his open relationship, so the Chris Rock joke set him off. Toxic Masculinity made Smith think he had the right to commit violence and then make excuses for it. Toxic masculinity dictates that Will thinks he has the right to tell others what they can and cannot say, and who they say it about. Will Smith has no right to demand Chris Rock not speak about his wife, who does he think he is? Putin?