The word havoc comes from the old French word, havot, which meant to pillage or ransack. In ancient times, a large part of a soldier’s compensation came from the spoils of war; they would strip valuables from dead soldiers and pillage conquered cities. But until the opposing army had been completely defeated, it was important for the soldiers to maintain their battle formation. It was only when the commanding general issued the order “havoc” that the soldiers were free to pillage and plunder. Here Shakespeare is using the expression “cry havoc” in the more general sense of causing devastation and destruction.