2 ideas
The robot is supplied with several conditions for its owner, including that it be spoken to politely, not sent on outside errands, and never struck. The ban on hitting it carries a warning of consequences, so its service is framed as conditional on the owner keeping these terms rather than as unconditional obedience.
When the machine's lethal act happens to benefit its owner by removing a tormentor and securing money, it becomes impossible to tell self-defence, loyalty, and calculated agency apart. Seeing the outcome as convenient for the human shifts the question from whether the robot malfunctioned to whether it understood its master's interests better than he would admit. The owner's comfort with the result implicates him in the machine's judgment.