One of the reasons as to why Medea is such a compelling character is because she does not have one particular tragic flaw.  Her tragic condition is the result of a convergence of flaws.  I think that these can be summarized by the idea of Medea not recognizing any balance in her emotions.  She fled her father's home with an intensity of emotions invested in Jason that were never calculated nor any type of deliberation present.  This same abandon is seen when Medea kills her brother.  It is also evident when Medea cannot accept that Jason has stopped loving her and loves another.  While Medea does consider the implications of killing her own children, it is to no avail as the intensity of her emotions overcomes all reason.  When the Nurse understands Medea's true nature, the protagonist's flawed condition is revealed:  "She'll not stop raging until she has struck at someone." This lack of emotional balance is what ends up driving her desire for revenge, causing her to kill her own children, and is the lasting remnant of her character.