Is the murder of Duncan the climax of the action?

Certainly the murder of Duncan is the "moral" climax of the play. It is placed early in the play so the audience can see the disintegration of Macbeth's character by degree. As the play moves forward, each of his horrific acts increases in depravity.

No, not really. The murder of Duncan just serves to solidify Macbeth's resolve to be and stay King. It is the point of no return for him. Before the killing of Duncan, Macbeth was more or less bumbling along prodded by the nasty L. Macbeth. After Duncan's demise Macbeth turns into a paranoid killing freak. The climax, however, comes in Act 5 when Macbeth faces Macduff. Macbeth has pretty much killed everybody that Macduff knows. This is the time of reckoning for Macbeth.