All of the bystanders ran away. They didn't stand around and laugh at the guy. Nor did they seem to support the two people who set the guy on fire.
One person just happened to have a cup of flammable liquid to throw at the guy, while inside a building.
And another just happened to have a lighter, which was applied within a second of the guy being doused with the liquid.
The guy survived and knew how to minimize the damage to himself.
= = = =
I say it was staged by the Communists. Or at least the two guys who set him on fire were Communist sympathizers who had knowledge that this guy was going to make a scene.
"One person just happened to have a cup of flammable liquid to throw at the guy"
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The protesters do make firebombs and have used them on people, it doesn't surprise me that some would carry something flammable with them.
e.g. https://www.thesun.co.uk/news/10070000/hong-kong-protesters-off-duty-cop-firebomb
"The guy survived and knew how to minimize the damage to himself."
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There's no need to come up with tortured reasoning to avoid acknowledging that sometimes some people do things that range from the silly to the genuinely evil in the heat of the moment.