Depending on the particular circumstances, none of what he did is contrary to the Geneva Convention. You can destroy structures used by the enemy. You can establish free fire zones. And you can surely use a 50 cal.
""I used 50 caliber machine guns, which we were granted, ordered to use which were our only weapon against people.
From what I understand, he got the Bronze Star for his act of jumping from his boat and running down and killing a wounded VC who had fired a RPG at his boat. Are we to assume that since he said a 50 cal was the only weapon they were issued, and if I am correct, the swift boats had dual 50s hard mounted in a gun carriage, that the Senator carried this several hundred pound weapon ashore with him to dispatch the wounded VC? Not even John Wayne could handle that task. And if the VC was wounded by a massive 50 cal round, was he in any condition to to offer resistance? It is a war crime to intentionally shot wounded combatants who are not offering resistance. The Geneva Convention states that if possible, they must be taken as POWs, treated humanely, and given medical treatment.