At what point does Shakespeare's Macbeth become a tragedy?

Shakespeare's drama becomes a tragedy when Macbeth kills Duncan.  
I think that Macbeth's murder of Duncan is the point where the drama becomes a tragedy because it shows Macbeth doing something he does not want to do.  When confronted with the prospect of committing such a horrible act, Macbeth is uncertain.   He communicates this towards the end of Act I:

First, as I am his kinsman and his subject,Strong both against the deed; then as his host,Who should against his murderer shut the door, Not bear the knife myself.

Goaded by Lady Macbeth, Macbeth kills Duncan.  In my mind, this is where the drama becomes a tragedy. Macbeth, a noble warrior and powerful human being, is reduced to doing what he does not want to do.  It is tragic to see someone reduced to such a state.  
Macbeth's murder of Duncan  also marks how the drama is a tragedy because it is clear that nothing good can come from this moment.  Macbeth's deed is not one of construction.  It is not an act of restoration.  Rather, he sets in motion the chain of events that lead to his undoing.  Killing Duncan is where Macbeth's moral compass disappears. The struggle that he had in having to "bear the knife" reflects that he knows the difference between right and wrong.  In expressing his desire not to kill Duncan, Macbeth shows that he can be good.  However, this shred of goodness disappears when he kills Duncan. Macbeth ends up being incapable of going back to this potential for goodness. The permanent loss of Macbeth's decency is another reason why this particular moment marks the drama as a tragedy.