I like "double-edged sword" but I will offer a couple more ideas.

> ***bane*** a person or thing that causes misery or distress (esp in the phrase *bane of one's life*) ([Collins][1])

> ***affliction*** 2. something responsible for physical or mental suffering, such as a disease, grief, etc. ([Collins][2])

Examples of use:

> The scientist's brilliant mind turned out to be the bane of his existence.

> The artist's extraordinary sensitivity to beauty was, paradoxically, also his affliction.


  [1]: http://www.thefreedictionary.com/bane
  [2]: http://www.thefreedictionary.com/affliction