Macbeth's fatal flaw is that he is ambitious. As with all tragic flaws, the flaw itself isn't necessarily a bad or evil characteristic. That's what makes readers sympathetic to the hero character. Macbeth is no different. He dreams of improving his social status and gaining more power.
Shakespeare alerts his readers/viewers to Macbeth's ambition early on when Macbeth is given the title of Thane of Cawdor. That's good news to Macbeth, but then the flaw is exploited through temptation. The witches tell Macbeth that he will be king.
Now Macbeth has a problem. He's been told he will be king and he's ambitious, which means he's not patient. Add to that a wife who is equally ambitious and twice as ruthless and Macbeth starts down his slippery slope of murdering just about everybody.
Despite his ambition and decision to kill Duncan, Shakespeare still allows the reader to see Macbeth as a flawed character instead of a sociopath. Macbeth is hesitant to kill Duncan in the first place. In fact he tells his wife that he won't do it, but she berates him and questions his manhood until he agrees again. He feels super guilty after having killed Duncan too. Shakespeare makes you really feel like it's always within Macbeth's power to put a stop to all of it, yet just can't quite do it (because of that flaw).