The little weak ork just took his Cutty-knoif, a weapon he looted afte battles against the Rising Sons and the Tau, and clearly cut his warboss' head in front of everyone, making his severed neck a fountain of greenblood the little ork was covered in.
The surprise of all the waaagh was followed by a loud WAAAGH across the planet. If a proper warboss couldn't defend himself against a little weak and wounded ork, he didn't deserved to be warboss at all. The new WAAAGH field generated made him grow, even bigger than his original boss.
His first order was to spare the rest of the humans on the planet, and them his plan was on: Prostitution, drugs, alcohol, illegal casinos.
His plan was simple: Keep them afraid, keep them happy, keep them busy. With those three bases, he secured his control over the planet, preventing riots, keeping them working like grots in the maintenance of the hiveworld.