The player clearly _didn't_ just want to attack whoever 'attacked' him - since they were _all_ going to attack him. It was quite clear that he wished to attack whoever actually managed to _hit_ him.
The problem then is not misinterpretation of the player's wishes by the GM, but rather the player not understanding that his desired action made no sense in terms of the 'readied action' rule.
Therefore a kind GM should explain that there was no point in him readying an action like that, he should have attacked normally, then after surviving foes had attacked him, he could then have chosen to attack whichever ones actually hit him.
That's actually far kinder to the player than misinterpreting him to mean a readied 'I attack whoever attacks me', I think, especially if he had iterative attacks.
The problem then is not misinterpretation of the player's wishes by the GM, but rather the player not understanding that his desired action made no sense in terms of the 'readied action' rule.
Therefore a kind GM should explain that there was no point in him readying an action like that, he should have attacked normally, then after surviving foes had attacked him, he could then have chosen to attack whichever ones actually hit him.
That's actually far kinder to the player than misinterpreting him to mean a readied 'I attack whoever attacks me', I think, especially if he had iterative attacks.