Adam knew they had badly misunderstood him. He had never had any intention of letting them go; the second half of that sentence was only, "...I can make your deaths a bit quicker."
Since the test subjects refused to cooperate, Adam had no objection to cruder methods. Strictly speaking, a master-servant contract did not require the consent of both parties at all — all it required was that the dominant party be stronger than the one being dominated.
Adam lifted the devil's speech restriction, then crooked a claw and drew Carter, the nearest to him, over to his side. A sharp talon pierced his forehead.
The devil was thrilled. It felt it was realizing its devil-life's value, one step at a time.