Macbeth is his own worst enemy. A really good example of this is after he kills King Duncan, and instead of sticking to the plan, he kills the guards. “Oh yet I do repent me of my fury/that I did kill them!” (Act 2/sc 3, line 107) Macbeth panics, thinking that leaving the guards alive will somehow come back to haunt him. In reality, this is the moment when his friends first start to suspect him. Eventually, their suspicions are confirmed and they rebel against him.
Another example of Macbeth’s actions leading to his downfall is the decision he makes to kill Banquo. Banquo is his best friend and the only person to have witnessed the predictions made by the witches. Once Macbeth and Lady Macbeth carry out their plan, Macbeth begins to think that Banquo’s honorable nature will prevent his friend from supporting him as king. “Our fears in Banquo stick deep/And in his royalty of nature reigns that which would be feared.” (Act 3, Sc. 1, lines 49-51) At the end of this important soliloquy, he has decided to kill his friend. “Rather than so, come, fate, into the list, and champion me to th’ utterance” (lines 71-72). The reader can perhaps understand his ambition leading him to kill Duncan, but the murder of his best friend is much harder to justify. His inability to try to protect himself and his crown at any cost is what leads to his downfall in the end.