"Now imagine the same situation but this time the gentelman is fitting terrorist profile and arguments get heated and there is a potential threat to me (an innocent bystander). Than I surely will do something about it."

Why would it make a difference if he fits a 'terrorist profile', aka looks middle eastern or muslim? An actual terrorist would never be stupid enough to draw attention to themselves by harassing a flight attendant.

In any case, if someone is abusing a flight attendant or anyone else, I think we have an obligation to intervene.