The Nippon Airways passenger who went nuts on a flight may have been suffering from PTSD ... so claims the former Marine who kicked his ass.

Ryan Humphreys -- the passenger who won the brawl -- tells us the aggressor was crazy talking about the U.S. government and Vegas, and also threatening to kill him. Humphreys kept his cool until the other passenger started smothering a woman in front of him, and that's when fists started flying.

HOW IT WENT DOWN
TMZ.com

The other passenger was thrown off the plane in Tokyo and then arrested after he allegedly put a choke hold on an airline employee.

Humphreys declined to press charges which makes sense ... they would have been delayed 4 hours had he done so and besides ... he really gave the guy an ass whooping.