Macbeth admits to killing King Duncan's guards after seeing Duncan's body.
Due to the fact that Macbeth and his wife first killed King Duncan's guards, nobody could see what they were about to do. Then they proceded to murder the king himself. After his body was discovered, Macbeth confessed he had killed the guards because he thought they were the culprits for Duncan´s killing. He explained he was struck with grief and had to kill them because of their alleged murder.