Over the course of the play, we witness Macbeth's rapid descent into almost pure evil. Shakespeare presents Macbeth at the beginning of the play as a good man, a "worthy kinsman" according to his cousin, King Duncan. Macbeth is "good" (or at least Duncan thinks he is) because he is loyal to the King and steadfast in carrying out his duty as a thane to Duncan. He is contrasted with the wicked rebel Macdonwald, who he is described as having killed in battle, and the thane of Cawdor, who is executed as a result of his treachery.
Just before he kills Duncan, we see at least some good in Macbeth. He hesitates before the murder on the grounds that there is nothing but "vaulting ambition" that drives him to do the deed. He realizes that the murder is especially evil because Duncan is his kinsman, his king, and because he is honor-bound to provide the man with hospitality in his castle. So violating these ties is portrayed as evil. Macbeth has both violated his obligations to his lord and relative and attempted to transcend his place in society by usurping the crown. By the end of the play, Macbeth has become a figure of almost unmitigated evil, having Banquo and Macduff's family murdered in an attempt to keep his crown.