They were killed by Macbeth because he blamed them for Duncan's murder.
"I" said Macbeth, "with the servants' daggers. I killed Macbeth."
And then Macbeth asked Macbeth if he confronted Macbeth on killing Macbeth with Macbeth.
Macbeth stabbed him when he was asleep.
Duncan is his cousin. Duncan is his king. Duncan is his guest. If he murders Duncan, he invites others to murder him. Many people like Duncan for being a nice guy and will be angry when he is killed.
He says he was overcome with rage because they had killed Duncan.
In shakesperes story it is said that Macbeth killed him in his sleep but in reality historians believe he was killed (by Macbeth) during or shortly after the battle.
Macbeth kills Duncan in his own castle at night time.
He had his suspicions because the fact the Macbeth killed the servants he blamed the murder on.
Macbeth was a loyal vassal of King Duncan. Macbeth's act should not be considered regicide.
The title character, Macbeth, Thane of Cawdor.
Macbeth said that he killed Duncan's grooms out of anger over Duncan's death.