Trump is free to have his opinions about McCain and frankly I don't care. I didn't vote for Trump because he was a quiet man who tip toed around everyone's feelings. I voted for him because he was aggressive, blunt, and unafraid to fight the battle. I find his comments distasteful but if he had every right to say them. I'm far more concerned with his ability as a leader and his accomplishment in office.
Being a vet doesn't make you infallible to criticism and neither does being a POW. In the military we know when to call people out or their actions and words. McCain in his later years was doing troubling things and things that gave him a negative reputation. Service does not erase that.