Macbeth's sentiment is expressed in scene iii, soon after he and Lennox have returned from the murdered king Duncan's chamber. They have witnessed the bloody and gruesome result of a most pernicious crime. Macbeth cries out:
Had I but died an hour before this chance,
I had lived a blessed time; for, from this instant,
There's nothing serious in mortality:
All is but toys: renown and grace is dead;
The wine of life is drawn, and the mere lees
Is left this vault to brag of.
He states that if he had lived only up to the moment that he discovered Duncan's bloody corpse, his life would have been blessed, but now, life has become meaningless and everything has lost its true value. In referring to Duncan, he mentions that prestige, honor and elegance have all died with him and cannot possibly exist. What makes life meaningful is now gone and they are left with only the dregs of what made Duncan's presence and their lives purposeful.
Macbeth's comment is a form of situational irony. Irony is found in the fact that the context in which he makes the statement makes it quite clear that he does not, in reality, wish to be dead at all. He actually wants to be more alive than ever since it is he (assisted by his equally heinous wife) who has assassinated the king so that he may claim the Scottish throne. In having succeeded in the first part of this malicious venture, he has achieved a major goal in achieving what he calls his "overriding ambition."
Secondly, stating that if he "had died an hour before" is dramatic irony since it is he who committed the crime. When he and Lennox went to Duncan's chamber, he already knew that Duncan was dead. Lennox obviously does not know this and Macbeth's remark is clearly made to impress upon them how deeply distressed he is about his king's untimely death. The dramatic irony lies in the fact that the audience knows what he has done and that his so-called trauma is nothing but a blatant lie said only to mislead and confuse.
One may also assume that Macbeth does, at this point, feel some regret for what he has done and does realize to a certain extent that his actions would lead to greater distress. He had previously refused to carry through their pernicious plot but was persuaded by his wife to persevere and take it to its ultimate conclusion. It is quite ironic that Macbeth's words, in this instance, foreshadow what is to transpire later. His life does indeed become meaningless, for he is so tormented by guilt and remorse later that he cannot sleep and actually wishes to die, as he states in Act V, scene lll:
...I have lived long enough: my way of life
Is fall'n into the sear, the yellow leaf....