tragic flaw

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A flaw in the character of the protagonist of a tragedy that brings the protagonist to ruin or sorrow.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A personality trait or other characteristic of a real or fictional individual which is immoral, destructive, or otherwise faulty and which leads to the ruin or profound suffering of that individual.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the character flaw or error of a tragic hero that leads to his downfall