The magistrates of Manchester, watching the assembly from an upstairs window, panicked and sent a troop of undisciplined and possibly drunk Yeoman Cavalry into the crowd to arrest Hunt before he could speak. The cavalry, composed of the sons and social peers of local mill owners, decided to confiscate the banners from the assembly, and they pushed their way into the tightly-packed crowd, hacking with their swords. Mayhem ensued, hundreds were injured and at least 19 were killed.
The melee became known as "Peterloo" in a mocking reference to Waterloo, contrasting the soldiers at Waterloo with the brutality of the Yeoman cavalry who attacked defenseless people.
Speaking for myself, I am more interested in understanding the point of view of the authorities than I am in condemning them. Plus, I wanted to represent the views of both sides in my novel, not create a one-sided piece of agitprop like Mike Lee's Peterloo movie of 2018.