When promoting games like this the media will as common practice invite a few ex-players to talk about how hated you are when playing at the home of your rival, then they'll big up the game by calling it "Super Sunday" and using phrases like "in this City, nothing means more than this." It's that sort of context that has contributed to this idiot trying to make himself a hero so I think the right thing would be to step back from the emotive language and treat him as a common criminal - I can't see why he won't end up before a judge to try and explain himself.
Would Grealish have been charged with GBH if he had reacted? Looking to the history books and Roy Keane famously ended a Man City players career with a deliberate horror tackle and I can't remember him getting any punishment outside of football. Grealish did the right thing by firstly not getting involved and then scoring the winner.