Your distance measurements seem a bit inaccurate.
I think if he has a good lawyer, he'll be able to convince a jury that the flames constituted an extreme threat to the life or health of those being attacked with the flames, and that the use of deadly force was justified. His firing of a shot that was clearly intended as a warning shot, deliberately aimed at the ground immediately after Mr. Long directed a burst of flames toward the rival protest group could be portrayed as an act of extreme restraint.
Of course, he probably can't afford a good lawyer.
In all seriousness, I do think he will try to portray his actions as defensive in nature, protecting his friends from a deadly threat. I don't think it will work, and I don't think it ought to work, but I do think that Mr. Long's actions with the makeshift flamethrower are a good demonstration of the sort of escalating violence present on both sides that were present at that rally. It was a powder keg that was primed by both sides. I do think that Mr. Long should have been arrested, but perhaps that's unfair. Perhaps he stands out among a huge crowd of violent people just because of the novel nature of the weapon he employed.