A group of space criminals collectively known as the Butchers is gathering enough power to be considered as a serious threat. Although they are nowhere near as powerful as the two major space empires, they are large, heavily armed, and able to be a serious thorn in the sides of the two major space empires.
The Butchers are composed of four separate groups.
Pirates raid other ships that are already in space. They prefer convoys that are already in deep space, and are very adept at capturing valuable ships, repairing them, and either reselling them or using them themselves.
Raiders are similar to pirates, but instead of attacking ships, raiders attack settlements. There they take supplies, weaponry, food, water, and anything else they want.
Outlaws are people or groups that are wanted by either of the other space empires and have decided to live outside of the law. They live with very few rules and tend to be excessively violent.
Slavers either buy captives from the other three, or directly attack ships or settlements on their own to capture people. They turn their captives into slaves for use or sale.
All four groups are ruled by the Butcher King, a former general from one of the space empires that was removed from command because of his extreme brutality. He is personally motivated by his hatred for the rest of civilization, and uses the Butchers to harass the less protected elements of each empire, while also attempting to cause tensions between the two to escalate until full scale war breaks out.
The question here is what holds this group of criminals together? Obviously the threat of violence from the rest of the group would be effective, but it seems like that alone would fall far short of keeping this group unified enough to be a serious threat to two technologically and numerically superior foes. If it helps the Butchers have access to faster-than-light travel, drones, and AI, but all of it is at least slightly inferior to their more civilized counterparts.
