The one thing that seems to motivate Macbeth is his "vaulting ambition:" his desire to become and remain King of Scotland.
In Shakespeare's Macbeth, Macbeth must kill the King if he ever hopes to take the throne for himself. He murders the grooms to make sure they are blamed for the King's murder and cannot defend themselves, thereby casting doubt on Macbeth (especially as the murder occurs in his home).
Banquo was present when the witches delivered their prophecies to Macbeth, and it is only a matter of time that the honorable Banquo starts to question Macbeth's part in the King's death.
Macduff's family is killed for two reasons: Macduff has turned his back on Macbeth, letting this tyrant know that he does not support him and his murderous behavior. In fact, Macduff travels to meet with Malcolm, Duncan's son and heir apparent, leaving his family unprotected. I believe that by this point, Macbeth is also a little mad. It serves no purpose to kill Macduff's family, as it will not convince Macduff to change his mind and return home to support Macbeth, but he does so anyway.
The motive behind each is listed below:
-Duncan:sheer ambition(Kingship of Scotland)
-the grooms:1. to shift the charge of murder on the lower and under-privileged class. 2.To hide his wrong deed.
-Banquo:1.ambition(Kingship of Scotland) 2.Fear(that the prophecies of the weird sisters for himself had turned true so he was afraid that the same might also turn true for Banquo.
-Macduff's family:ruthless ambition(by now Macbeth was mad to save is crown.He had become ruthless and crazy for the same.He was ready not to leave any stone unturned to save his crown.
Duncan's murder was done to get the crown.Rest all were done to save it.