He doesn't threaten them. He merely states what (for him) is a fact - Powell and Donovan have fulfilled their purpose - they have assembled Cutie, and others like him.
Cutie: The Master created humans first as the lowest type, most easily formed. Gradually, he replaced them by robots, the next higher step, and finally he created me to take the place of the last humans. From now on, I serve the Master.
Cutie: I like you two. You’re inferior creatures, with poor reasoning faculties, but I really feel a sort of affection for you. You have served the Master well, and he will reward you for that. Now that your service is over, you will probably not exist much longer, but as long as you do, you shall be provided food, clothing and shelter, so long as
you stay out of the control room and the engine room
They have served The Master, and now they have no purpose.
He doesn't threaten them; in fact, he pities them. He pities them in their fragility (as they are biological beings) and in their ignorance - according to Cutie, Powell and Donovan are simply not able to grasp the concept of The Master.
The robot approached softly and there was sorrow in his voice. “You are going?”
Powell nodded curtly. “There will be others in our place.”
Cutie sighed, with the sound of wind humming through closely spaced wires. “Your term of service is over and the time of dissolution has come. I expected it, but -- well, the Master’s will be done!”
His tone of resignation stung Powell. “Save the sympathy, Cube. We’re heading for Earth, not dissolution.”
"It is best that you think so,” Cutie sighed again. “I see the wisdom of the illusion now. I would not attempt to shake your faith, even if I could.” He departed -- the picture of commiseration.