Duncan, King of Scotland was actually crowned in 1034 AD and ruled for 6 years. In The Tragedy of Macbeth, there are a number of characters who could be held accountable, and liable for his murder!
Shakespeare has Macbeth's castle at Dunsinane Hill, Perthshire in central Scotland, just south of the Highlands where there are still remains of two early forts.
Good King Duncan is a guest, sleeping at Macbeth's castle. Macbeth follows the image of a dagger to Duncan's bedchamber and kills him in his sleep.
This evil deed is only implied and happens off stage in Act 2, Scene 2 when in performance but there are more people involved in the death of the King, other than just Macbeth, Thane of Cawdor!
KING DUNCAN
Duncan is a weak, niave, incompetent, complacent, king. He foolishly trusts way too easily and does not learn from his mistakes.
For example, his first Thane of Cawdor betrays Scotland by fighting for the Norwegians against him and then history repeats itself as Duncan's second Thane, a.k.a. Macbeth, in return for promotion and all the land and title associated with Thane of Cawdor, kills the king for his trouble!
Duncan cries half way through a speech, which put his guard down. A strong king should always control his emotions and perhaps this show of weakness opened the door for an assassination attempt.
He should have hired more professional minders that could do their job properly and not be easily led by Lady Macbeth, getting drunk instead of guarding their king.
Duncan practically commits suicide with all these blunders. His decision-making brings on his own death making him his own worst enemy and is thus, partly responsible.
GUARDS
The bodyguards were supposed to protect him. Think what an outcry there would be these days if our monarch were bumped off by a rival who gained access through those trying to protect by drugging them.
We have seen Prince Charles minder in action in the past. I am sure there are terrorists out there who would like to manipulate, drug and tap into the royal family of England via their round the clock team! Our Boys do their job properly, quite clearly and so should have Duncan's.
BANQUO
He knew of the witches' prophecy and Macbeth's desire to be king but did not warn Duncan. If he had, they could have formed an alliance and got Macbeth first, saving both their bacons!
Banquo may have been jealous at the lack of reward from King Duncan for services to Scotland. He got just a hug, whilst Macbeth got land! Banquo may have turned a blind eye to the Macbeths' ambition, but his knowledge of a pre-meditated murder, does make him part of the plot.
WEIRD SISTERS
Their unarranged appointment on the heath and subsequent prophecy tells Macbeth he will be king. He was happy not to be king and probably would not have killed Duncan without this chance encounter.
James 1st was interested in witchcraft. Perhaps this is why the witches are included though how such claims about prophecy would stand up in court these days, and how you would judge a supernatural being or three in court, I do not know. As far as the play is concerned The Three Witches are the first to set Macbeth on his path of destruction, and must bare some of the responsibility for the death of King Duncan.
LADY MACBETH
She provokes her husband by questioning his manhood having bottled it herself because Duncan resembles her late father. She wanted to be queen more than Macbeth did king, and drugs the guards protecting Duncan, which makes her part of the conspiracy and just as much a murderer as Macbeth.
MACBETH
"I have done the deed. Didst thou not hear a noise?" Hallucinations, bell ringing and knocking of the gate, in Act 2, all allude to the offstage killing of the King. It is MACCA who appears with blood on his hands and finds it difficult to wash off. You would not need a forensic scientist to deduce that it was not the chamber men!
SUMMARY
By the time he wrote Macbeth, Queen Elizabeth had passed and James 1st had moved down from Scotland to London to take control. Shakespeare's company was renamed the Kings Men in response to the new arrangements.
King James himself had experienced an assassination attempt. In fact, his father was assassinated like Duncan and his mother was executed. James was nearly killed by the infamous Guy Fawkes in the gunpowder plot of 1605.
This theme of usurping the throne was hotly on peoples minds at the beginning of the Stuart period, including the new king and the poet himself.
Duncan 1st was actually killed by his own men, led by Macbeth in battle in 1040 at Pitgaveny near Elgin, which is north of Aberdeen and just east of Inverness!