Someone just posted photos over a crowd trying to save the people in that building. So “left to burn alive” is somewhat of a stretch.
Oh horse hockey
You can watch the videos and on the same film clip whoever shot it points the camera one way and there are people helping those in the building escape then points in another direction and there are uniformed police shooting at the people trying to escape. Point somewhere else, and there are police protecting the people escaping from the mob trying to get at them and point in another direction and there are people kicking and beating people who escape.
There clearly were elements in the mob that wanted to let the people in the building burn to death.