A regime can only be as oppressive as the people following its orders. There are only so many protestors you can order your army to kill until your troops realize they are killing their own friends and family, at that point they either desert or turn their guns against you. This puts a check on what even the most evil tyrants can do. Not a perfect one, for sure, but still better than nothing.

Technology changes this by reducing the human involvement needed in killing. The Nazi gas chambers were a historical example — it turned out mowing down innocent civilians with machine guns was too mentally taxing for even devoted Nazis, so they invented a bloodless industrialized process where they can kill with greater efficiency while also sheltering the killer from the immediate effects of his actions. A win win for the Nazis, not so much for its victims.

Autonomous weapons are dangerous for this exact reason. They bypass individual moral decisions entirely. They are the perfect "just following orders" machine. They allow a small concentration of powerful people to impose force upon the populace without ever having to fear their own soldiers. It tips the balance of power massively in favor of those already powerful.