Why did Macbeth not feel successful after killing Duncan?

After killing Duncan, instead of feeling successful, Macbeth becomes guilt-ridden and mentally disturbed. He begins to hallucinate, seeing the ghost of Banquo, whom he had also ordered to be killed, and becomes increasingly paranoid about the threat of his own downfall. Rather than feeling a sense of accomplishment or satisfaction, Macbeth becomes consumed by his guilt and the consequences of his actions.