Macbeth at first seems a somewhat sympathetic character, if only because he knows initially that what he is doing is wrong. In other words, he is not a psychopath: he has some moral sense. If he were a psychopath, we would not sympathize with him. If he were a total victim, he could not be a tragic figure. Instead, he is a complicated human being with a complex moral nature, although by the end of the play he becomes one of the least morally attractive of all of Shakespeare's tragic figures.
In order for Macbeth to truly be a tragic hero, he must be a man of greatness to begin with, and he is. In Act I, for instance, the captain relates the acts of "brave Macbeth" as he fought the "merciless Macdonwald." And, as a tragic figure, Macbeth falls from greatness, a fall that is always cause for sympathy. For, he has been, indeed, a noble warrior; he has truly loved his wife and respected the king Duncan. But, his "vaulting ambition," of which he is aware, has wrought his destruction.
All real tragedy, Aristotle contends, effects a purging of emotions, and the tragic hero's misfortune is not wholly deserved. Therefore, the reader/viewer of Macbeth does feel some sympathy for the main character.
Considering whether or not to feel sympathy for Macbeth can be based upon whether or not you think Macbeth has any control of his own destiny.
If you thik of Macbeth as a pawn, then you can feel sympathy. He is controlled by the witches at exactly the rigth time when he has just been "promoted" to Thane of Cawdor. The witches give him something else to strive for. Macbeth is also controlled by his agressive and demanding wife. He has no choice but to follow her plan. In these instances, you can feel sorry for Macbeth because he is unable to make good decisions because of the pressure from the witches and his wife.
However, if you think Macbeth is simply irrational and power hungry then he gets what he deserves in the end, no sympathy for the continuous bad choices he made. He was doing well by simply supporting and fighting for King Duncan and Scotland. Who knows what would have happened had he followed that path. Hard to feel sympathy for a character who continually makes bad choices.
When questioning the sympathy one has for the protagonist in Shakespeare's Macbeth, one must realize that many answers will be different based upon personal relation to the character.
That being said, one could certainly justify their sympathy for Macbeth. Macbeth is pushed by a demanding wife, promised the crown by the witches, and conflicted by his own feelings about ambition and what is right/wrong. Therefore, one could certainly state that they feel sympathy for him based upon how he is treated by other characters in the play.
Outside of that, one could also argue that Macbeth made the choices about trying to change his fate. He was the one who went through with the murders and claiming of the crown. If one were to see Macbeth in this light, they could certainly say that he deserved what he had coming.