What Macbeth means is explained by what follows. He says: "Tomorrow and tomorrow and tomorrow / Creeps in this petty pace from day to day / etc." and concludes with a famous metaphor, that "It is a tale told by an idiot, full of sound and fury, / signifying nothing." Life, according to Macbeth, is totally meaningless, so it doesn't matter in the least whether anybody dies today or tomorrow. The word "should" does not mean that he thinks she shouldn't have died so soon but should have lived longer and died "hereafter"; the words is somewhat archaic and means the same in this context as "would." She would have died sometime, so what difference does it make whether she dies now or dies later? What difference does it make if anybody dies now or hereafter? Macbeth is acutely depressed and is expecting to die himself in the near future. He really doesn't care.