Macduff's diary - regicide?

Misaki has finished reading her first Shakespeare, and really enjoyed it (despite all the bloodshed). Macbeth can be heavy going! As a follow up to the book, Misaki was asked to write Macduff's diary for the day of (King) Duncan's murder:

That morning I arrived at Macbeths's castle.

At the door I met Macbeth's servants. He said "Everyone is sleeping, I think. Shall I wake the general?" At that time, Macbeth came downstairs.

I said to Macbeth, "Macbeth! Good morning my friend." And I asked him where the King was.

Macbeth and I went to the King's room. I went into the King's room and saw that the King was dead! He was killed by someone. This is murder!

All of the people who were in Macbeth's house were surprised, and Lady Macbeth cried out and fainted.

Who did kill the King? I want to know the truth.