Why did the “sans culottes” engage in the September massacres? Who was killed? What was the international reaction?

Sans Culottos were militias of lower class Frenchmen in 18th century.

Explanation:

After and during the French revolution these militias known as Sans Culottos which consisted of common people and lower class serfs came to dominate the landscape as they had a free run to kill who they wanted to amidst the angst against the upper class.

This was something that was condoned as they were primarily killing landowners and people from upper class who had slighted them.

Internationally, feudal got afraid that this could happen in their country too.