Question

Why does macbeth claim he killed king duncans guard

Answer 1

Answer: because he became angry after seeing king Duncan covered in blood

Answer 2

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