What is Macbeth's view on the nature of justice?

Alone on stage, Macbeth agonizes over whether to kill Duncan, recognizing the act of murdering the king as a terrible sin. He struggles in particular with the idea of murdering a man—a relative, no less—who trusts and loves him. He would like the king's murder to be over and regrets the fact that he possesses “vaulting ambition" without the ruthlessness to ensure the attainment of his goals (27). He wonders whether justice will come in this life or the afterlife.