Why was Macbeth crowned king after the death of King Duncan?

Macbeth plotted the murder of King Duncan and gives the impression as his guards who got drunk had killed him, later he kills the guards showing how angry he has become with the murder of King Duncan.

However Malcolm the son of Duncan seems to know the reality behind the incident but he fled the country leaving the throne vulnerable.

Macbeth is the only eligible choice to be crowned as the king of Scotland as he had previously fought bravely in the battles. But his mischief did not stop here, the story of his atrocities continued.