"I fear Banquo deeply. There's a Kingly quality in his nature that should be feared" (III.i.52-54).
"He is my enemy as well. And he's such a close and deadly enemy that every minute he lives stabs at my heart (III.i.27-28).
"The grown serpent lies there ; the young one that got away will grow up to be poisonous"(III.iv.32,33).
"Or come to life again and challenge me to fight you in the desert. If I tremble then, call me a baby girl"(III.iv.120-123).
"This drop distilled with magic skills, shall produce visions of spirits that, by power of illusion, shall bring him to destruction. He shall reject fate, scorn death, and place his ambition above wisdom, virtue, or fear. As you know, overconfidence is humanity's greatest enemy"(III.v.25-29)
Macbeth's descent into evil is mostly caused by his constant fear. He is somewhat skitsofrantic which causes his impulses to overthrow his rationalization. These impulses are what caused him to murder the family of Macduff, which will influence Macbeth's future in many ways, including leading up to his downfall. His actions create a panic in his kingdom, which makes finding a successor for Macbeth an urgent topic. The ghost of Banquo that haunted Macbeth only worsened the fear within Macbeth and made him feel as though he was being mocked and looked down on. His actions allow people to relaize that things are mot always as they appear.