Macbeth was once the general of King Duncan, the King of Scotland. But after hearing a prophecy from three witches telling him that he would one day be king, his wife told him to accelerate his ascension to the throne by killing King Duncan. After this plan succeeded, Macbeth started a murderous rampage, killing anyone whom he deemed a threat to his kingship. He was eventually killed by Macduff as revenge for the death of his family.
In the opera adaptation, Macbeth is a baritone.