he killed the guards after he had killed duncan
lady macbeth was sure at the time that "a little water clears us of this
deed" but after killing duncan macbeth went into a state of shock and
had brought the bloody daggers back with him. After refusing to go back
there, lady macbeth went and smeared duncans blood on the guards hands
and put t he bloddy dagges with them.
therefore in the morning, it appeared that the guards had comitted the murder.
Macbeth killed the guards, because it meant that they could not testify
that they had not done it, and it was alot easier for people to blame
the murder on the dead men, than if macbeth had not killed them and then
in turn they may have suspected him.
I found this online but i hope this helps
