In the play Macbeth, why does MacDuff suspect Macbeth so much ?

This sounds like an english homework... but anyway Macduff suspects him because Macbeth is slowly losing his mind, and also dont forget, Dunken died at Macbeths house. He was the only significant man in the house, in those days women were like an object to be owned. So macduff wouldnt of suspected Lady Macbeth. And if you dont know what im talking about Romeo and Juliets a good idea when Juliet refuses to marry paris. Anyway yeah... Macduff suspects macbeth so much because of his behaviour afterwards. Eg. at the table when he sees the ghost of his friend who he had to murder from becoming an inconvenience.