When the lords go to arrest Duncan's guards, they discover that Macbeth has killed them. He says it's because he was so angry with them for murdering Duncan, but it looks really suspicious. Duncan's sons are scared that they might be next on the hit list, so they run away. Soon after, Macbeth is made King of Scotland.
Macbeth kills Duncan because the witches first give him the idea, his wife gives him the plan, and both (idea and plan) reinforce his blind ambition to be king.
