Each tragedy will have a tragic hero. Each tragic hero will have a tragic flaw. More often than not, this flaw is hubris. Hubris is just a fancy way of saying pride. His pride will ultimately be his downfall.
For example, in Macbeth, while Macbeth does end up getting to become king like the three witches foresee, he is ultimately killed in the end because he doesn't believe their final prophecy. He's told "when Birnam Wood moves on Dunsinane," which sparks him to order his soldiers to cut the trees down. Yet, he still thinks he is safe. There are other things that happen throughout the play that he tries to circumvent by various means. They all circle back to Macbeth's hubris though because he thinks he is too good to be killed in the way he was told.
