Fear 

The underlying motives of  the upper classes and leading to the Peterloo Massacre, was fear. It was a starving French populace that had started the French Revolution leading to large numbers of upper and middle classes being sent to the Guillotine.

Here were the same circumstances of a hungry population getting very restive. Who knows where such a situation might lead, a mass hanging of the upper classes? Such gatherings as that at Peterloo had to be crushed with immediate effect, militarily if neccessary.