Final answer:
Macbeth kills the guards in an impulsive act of supposed loyalty to King Duncan, which was not part of the original plan concocted with Lady Macbeth, thus complicating their plot.
Step-by-step explanation:
When Macduff discovers that Duncan has been murdered, Macbeth, who was not supposed to reveal any complicity, kills the guards in what appears to be a fit of rage. This act was not part of the plan he had with Lady Macbeth, who intended for the guards to be blamed for Duncan's murder while being incapacitated by alcohol, thus appearing innocent themselves. Macbeth's impulsiveness disrupts their scheme by removing the intended scapegoats and casting suspicion on himself when he tries to justify his actions as being an overwrought act of loyalty to the king.