For both Macbeth and Othello, Shakespeare makes it clear that their downfalls stem essentially from character flaws. Macbeth's downfall stems from his excessively ambitious nature, while Othello's downfall stems from his irate temper and especially from his persistent feelings of distrust and jealousy. As we are limited in space, below are some ideas concerning Macbeth to help get you started.
At the opening of the play, we learn that Macbeth was very successful in battle during a civil war and defeated Macdonwald, the Thane of Cawdor, who raised an army from Norway and Ireland to challenge King Duncan. Once Macbeth defeats Macdonwald, King Duncan names Macbeth Thane of Cawdor instead. However, this one achievement and rise in station is not enough for Macbeth. The Three Witches ignite his ambitious nature by predicting that he will also become king. Out of his thirst for more power, he begins to plot killing King Duncan to gain the crown. We especially see his ambitiousness described in lines of his letter to Lady Macbeth:
Whiles I stood rapt in the wonder of it, came missives from the King, who all-hailed me "Thane of Cawdor"; by which title, before, these weird sisters saluted me, and referred me to the coming on of time with "Hail, King that shalt be!" This have I thought good to deliver thee, my dearest partner of greatness, that thou mightst not lose the dues of rejoicing. (I.v.5-11)
Hence, one action that leads to his downfall is accepting the Witches' prophecy and permitting their prophecy to influence him to the point that he begins imagining himself killing Duncan to win the crown.
However, Macbeth may not have gone through with the deed had he not been encouraged by Lady Macbeth out of her own ambitious drive. Hence, a second action leading to his downfall was accepting the encouragement of his wife.
Naturally, Macbeth's killing does not stop at King Duncan, nor do his fears. Macbeth also kills Duncan's servants after blaming them for Duncan's murder. Plus the Witches give Macbeth further prophecies about interferences in his keeping the crown. First they predict that Macbeth's friend Banquo will become king, so Macbeth hires men to have Banquo killed. Then the Witches predict that Macduff, Thane of Fife, will prove to be a threat to Macbeth, so Macbeth has Macduff's whole family killed. Killing Macduff's family actually proves to make Macduff a greater threat to Macbeth than Macduff might have been otherwise, for once Macduff's family is killed, Macduff swears to avenge himself on Macbeth. In addition, Macbeth approaches his final moment of downfall when fighting Macduff who vanquishes Macbeth and restores the crown to the late King Duncan's eldest son Malcolm. Hence, other actions that lead to Macbeth's downfall is his allowing himself to still be influenced by the Witches because it is the Witches' influence that leads Macbeth to commit even more murders. Plus, the Witches' influence leads Macbeth to his final battle with Macduff, which leads to Macbeth's demise; therefore, a final action that leads to Macbeth's downfall is having Macduff's family killed due to the Witches' influence.