We do not see Duncan's murder on stage. When Macbeth enters "with two bloody daggers", the reader/the audience will automatically deduce that the "deed" has been "done". This is how Shakespeare makes the reader imagine the murder in her/his own mind. This method had become quite popular in Shakespeare's times.
Look at the example of the famous contemporary Italian painter Caravaggio. His painting of the Beheading of St. John the Baptist
was also finished in 1608, just like Macbeth! Look closely at the right hand of the executioner. Look at the movement captured in the painting. It seems like a still from a (modern!) movie (in 1608!) which is being filmed in your own head. What are you going to watch in the next instant? Who will move his right hand to do what? What will be the effect? Isn't that a most frightening experience?
Also look at the perspective in the painting. It is no longer a central perspective, with the plot nicely arranged in the middle of the picture. Here, it is out of place (as is the deed!), moved to the left bottom corner and juxtaposed with two witnesses behind a window in the top right corner: A symbol of the looker-on who loves watching. Don't we all?
And, finally: what is in the centre of the entire painting? (Where do the diagonals meet?) How does that correspond to the title of the painting? Where, obviously, does Caravaggio (a convicted murderer himself!) place the main emphasis? And which stage direction in Act II, 2 reveals the murder - and identifies the murderer?