Macbeth's tragic flaw is ambition. This flaw helped him to power because he was able to use this blind ambition to do things like commit murder that would get him on the throne. Even when he is having doubts, his ambition leads him forward. He says, "I have no spur To pr**k the sides of my intent, but only Vaulting ambition" (1.7), which means that the only thing leading him forward is his ambition which is causing him to leap into the murder of Duncan.
This flaw ultimately caused his downfall because he wanted too much and went too far, led by his ambition. He tried to kill Fleance because he wanted his own children to be kings after him, and he killed Macduff's family on just the word of the witches that he should fear Macduff. He says, "And even now, To crown my thoughts with acts, be it thought and done: The castle of Macduff I will surprise" (4.1). This shows that he thinks he is acting out of a need to secure his position and stay in power, and that killing Macduff's family will help this. Unfortunately, it only spurs Macduff to fight harder and eventually kill Macbeth.