Macbeth comes to power through doing an evil deed. Macbeth murders the current king, Duncan. Macbeth and Lady Macbeth are hosting a party for the king in order to celebrate the recent victories in battle. While Duncan is asleep in his room one night, Macbeth sneaks into the room and murders Duncan. That's just plain evil. Macbeth killed an unarmed man in his sleep. Because King Duncan is dead, a new king must be found.
The king's two sons, Malcolm and Donalbain, are in line for the throne and should be made king. Unfortunately, the both run away after their father is killed. They fear that people will suspect them of the murder, and they fear that the murderer will come after them next. Since Macbeth is a high ranking noble and good military leader, Macbeth is made king; therefore, Macbeth comes to power through doing evil.