After the death of King Duncan, Macbeth immediately assumed the role of King and took the power and wealth for himself. Macbeth described that the guards bore the blood of Duncan, so he killed them while they were drunk. Macbeth also told Macduff and Banquo that the two sons of the king ran off to England and Ireland because they were afraid to be persecuted.
Well the reason Macbeth assumed the role of King is because he was next in line because the sons ran away and we can't leave the kingdom without a king. The sons feared for their lives so Macbeth told them to go somewhere safe. Macbeth was just hold the place until they found the kings murder.
After murdering Duncan, he had to cover up any evidence that he could be persecuted as the murderer of the king so in order for him to do so he killed the guards whilst they were still drugged and then smothered the blood on them and their swords.
Lady Macbeth forced Macbeth to hide all the evidence and to kill the guards. She broke him down mental until he was unstable. Lady Macbeth manipulated him into thinking that this is the right thing to do and that it needs to be done.
As a result of his desire for more power and wealth he persuaded two assassins to murder Banquo and his son, Fleance upon the return of their venture. They only succeeded in violently stabbing Banquo 21 times, then accidentally let his son flee. (Banquo's son testified and Lady Macbeth diary did as well)
All the son knows is that there were people there to kill his father and him. He doesn't know who they were because he didn't stay there to find out. Lady Macbeth is a sociopath so how reliable is she? She was the brains behind the operation and Macbeth was the muscle.