Macbeth's transition from a loyal and valiant subject into a traitorous, murderous tyrant conveys the idea that even someone who seems virtuous and good can be corrupted by ambition and pride. 

He begins the play as "brave Macbeth" who courageously fights and bests a rebel army bigger than his own (1.2.18).  When he hears from the Weird Sisters that he will become Thane of Cawdor and king, and then is named Cawdor, he hopes that the bigger title might just fall into his lap like the smaller one did.  He says, "If chance will have me king, why chance may / crown me / without my stir" (1.3.157-159). 

However, once Duncan names his son, Malcolm, as his heir, Macbeth realizes that he will either have to give up his aspiration for the crown or do something underhanded to get it.  Here, he says, "Stars hide your fires; / Let not light see my black and deep desires. / The eye wink at the hand, yet let that be / Which they eye fears, when it is done, to see" (1.4.57-60).  He asks the stars to hide their light so that no one will see the terrible things he's thinking.  He will not let his eye watch his hand, but he is still going to do the thing that would make his eye afraid to look.  In other words, he's resolved to kill Duncan, even though he knows it is wrong.

Once he and Lady Macbeth have finalized their regicidal plan, Macbeth lists a great many reasons he has not to go through with the murder.  However, he has one reason to go on.  He says, "I have no spur / To prick the sides of my intent, but only / Vaulting ambition [...]" (1.7.25-27).  His ambition overshadows everything else.  Almost. 

When Lady Macbeth enters, he tells her, "We will proceed no further in this business" (1.7.34).  Macbeth is sensible of all that he owes Duncan and has decided not to continue in their plan.  However, when Lady Macbeth repeatedly insults his masculinity, calling him a "coward" and insisting that he is not a man unless he goes through with it, he relents.  Ultimately, and for this reason, I believe it is his pride that -- when added to his ambition -- makes possible the tragedy of this tale.

The play isn't called The Tragedy of Duncan; it's The Tragedy of Macbeth.  This means that the titular character is a tragic one.  If he is simply evil from the outset, then there is no real tragedy.  Instead, the tragedy is that this once-great man was corrupted by pride and ambition to become a veritable monster by the play's end.