Who the hell would react by laughing and saying “what the hell?” when they see someone struggling to speak? I agree that they almost definitely didn’t have hurtful intentions. However, that shouldn’t be an excuse. Expressing concern would have been a more acceptable (though still hurtful in this case) response. Better yet, unless they see someone who is visibly in pain (choking, convulsing, etc), they should say nothing when they see something they don’t understand and mind their own damn business. OP responded perfectly. Next time maybe that person will think twice before being so thoughtless, and will be grateful to OP for the learning moment.