the reason we all love the punch so much is because it shows so much emotion. Charles Xavier is a man who loves everyone in a peripheral way, who doesn’t let people get close to himself and is in constant, careful control of his emotions. He has to be, it’s the only way to stay sane with a mutation like his. But Erik gets under his skin in a way that no one and nothing else does. Can you imagine Charles Xavier punching someone in the face, just generally? He’s the exact opposite of a violent person! But HE CARES SO MUCH ABOUT ERIK AND THAT’S WHY ERIK COULD MAKE HIM MAD ENOUGH TO DESERVE A PUNCH TO THE FACE
with a mutation like telepathy where to some extent you have to experience the world from other people’s perspectives, i would imagine it engenders a kind of apathy, if not a desire for distance from other people.
charles cares about erik enough to be completely involved in him. after xmfc, this is the fruition of their ideological divergence. that, combined with circumstances tbd in the movie, hurts charles. he loves erik enough to be hurt by their disagreements, to get angry about it, to take it personally and feel betrayed.
you can’t get that angry over someone you never really cared about.