In Shakespeare's Macbeth, is "decay" a theme or motif?

In this play, decay -- specifically Macbeth's moral decay -- is a motif.  A motif is a recurring symbol that has figurative meaning and contributes to the development of a major theme.  We see Macbeth's decay begin after the Weird Sisters hail him as Thane of Cawdor and the future king, and after he receives the first of these two titles.  He begins by thinking, "If chance will have me king, why, chance may /...

In this play, decay -- specifically Macbeth's moral decay -- is a motif.  A motif is a recurring symbol that has figurative meaning and contributes to the development of a major theme.  We see Macbeth's decay begin after the Weird Sisters hail him as Thane of Cawdor and the future king, and after he receives the first of these two titles.  He begins by thinking, "If chance will have me king, why, chance may / crown me / Without my stir" (1.4.157-159).  However, once Duncan names Malcolm, his son, the heir to the throne, Macbeth is willing to consider his own "black and deep desires" (1.4.58).  His declension has begun.

When Macbeth returns home, his wife is ready to murder Duncan, but Macbeth takes a bit more persuading.  He says to her, "We will speak further" (1.5.83).  Has has a lot of doubts, and he worries about the consequences of such a terrible action.  He finally tells her, "We will proceed no further in this business" (1.7.34).  However, once she insults his masculinity and wounds his pride, calling him "coward" and implying that he is not a man if he will not go ahead with this plan, he relents (1.7.47).  He finally says, "I am settled and bend up / Each corporal agent to this terrible feat" (1.7.92-93).  His morality rots away even more.

His guilt initially threatens to drive him mad, and he hallucinates prior to committing the murder and then panics afterward.  He seems to genuinely regret his actions.  However, it doesn't take long for him to decide to have Banquo and Fleance murdered.  He is remorseless until confronted by Banquo's ghost.  His moral decay seems complete with his murder of the innocent Lady Macduff and her children.  Macbeth has nothing to gain by their murders; he only commits them as a show of power and a point of pride.

Therefore, this motif illuminates the idea that unchecked pride or ambition can lead to the moral decay of a once-good man.