Why might the audience feel disgusted by Macbeth's behavior throughout Shakespeare's play Macbeth?

Although Macbeth is an intelligent man who seems to have upheld the morals of society up to this point in his life, the advent of the three witches’ prophecy that Macbeth will be king sets him on a morally downward spiral. Macbeth's gut reaction upon hearing the prophecy that he will be promoted to Thane of Cawdor and then become king is to justify to himself that this is not a bad thing.  As soon as Macbeth greets King Duncan in Act I, Scene 4, he fawns over the king, speaking of “The service and the loyalty I owe / ...by doing every thing / Safe toward your love and honor.”  In his heart, however, Macbeth vows to keep his “black desires” hidden. This takes “two-faced” to a whole new level.

To his credit, once the king is at Macbeth’s castle in Act I, Scene 7, Macbeth decides not to harm him.  After all, Macbeth reasons, the king is his cousin and trusts Macbeth as one of his thanes and military leaders. Macbeth feels Duncan has been a truly good king and, if he were murdered, all of Scotland and Heaven’s angels would mourn. The minute Lady Macbeth calls Macbeth “a coward in thine own esteem” for refusing to shed blood to become king, Macbeth caves and lets her manipulate him into committing bloody treason.

Although Macbeth's conscience haunts him, he stabs King Duncan in his sleep and lets his wife cover up the murder.  After this, Macbeth’s conscience seems to fade quickly.  Enjoying the power of kingship, his main concern is his competition—Banquo and his son Fleance—since the witches prophesy that the sons of Banquo are to be kings, not Macbeth’s.  In Act III, Scene 1, now-King Macbeth questions Banquo about his afternoon riding plans with Fleance, even telling his friend, “I wish you well on your journey.” Macbeth then sends three hired assassins to murder them.  That night at the banquet, knowing full well he’s had Banquo murdered, Macbeth says to his guests, “I drink to the joy of .../ our dear friend Banquo.../ I wish he was here.”  What a duplicitous hypocrite!  It is disgusting that murder has become Macbeth’s go-to option for any obstacle to his power.

Perhaps as expected, Macbeth and Lady Macbeth become rather distant from each other.  When his chief servant Seyton reports to Macbeth in Act V, Scene 5 that Lady Macbeth is dead, Macbeth's appalling response is that his wife should have died at a more convenient time.  Although Macbeth initially has some redeeming qualities, he systematically rejects each one. When Macduff produces Macbeth's severed head, the audience hardly mourns at all.