Why do you think Shakespeare decided to murder Duncan and his guards offstage in Macbeth?

This is a very thoughtful question, and I think there are
two main answers that can be given to answer it. Firstly, it is always the case that the
best horror movies are ones that do not show everything, but rather leave the work of
depicting the gruesome events that they show to the imagination, which is so much more
effective than any special effects. By not observing the murder of Duncan, we imagine
it, and often the imagination makes such events far more gruesome than they would be in
real life.

Secondly, and perhaps more importantly, by
focusing the action on Lady Macbeth and Macbeth outside during Act II scene 2, we see
their responses and reactions to this crime which help us develop our analysis of them
as characters. We see the nerves of Lady Macbeth and her moment of surprising humanity
when she tells us she couldn't kill Duncan because he resembled her father, but we also
see Macbeth's guilty response after killing Duncan, and how his wife treats
him:

Will all
great Neptune's ocean wash this blood

Clean from my hand?
No, this my hand with rather

The multitudinous seas
incarnadine,

Making the green one
red.

Here we see Macbeth's
incredible guilt and how he is haunted by his act. Basically, the focus of the play is
not on Duncan, who is at best a minor character. The focus is on the psychological
development of Macbeth and Lady Macbeth, and this is a vital scene to help us towards
analysing them both as characters.