Cool idea, although how about something more like this...
Bloodlust
This enhancement allows your melee weapon to damage a creature even when it misses. If during your attack you miss, you calculate damage without any bonuses except the magic enhancement, and evenly divide it between the wielder and the opponent. This enhancement only affects the base damage of the weapon and magic enhancement, but not any other bonuses named or unnamed. (No feats, Str, or spells, etc...)
Market Value +3 or so?
Fig6 (Str 18 (+4)) attacks Ogre with greatsword +2, Bloodlust. First attacks hits for 2d6+8. The second attack misses and drains 1d6+2 from Fig6 and does 1d6+2 damage to the Ogre.
Fig6 (Str 18 (+4), Power attack -6 to hit/+6 to damage) attacks Ogre with Greatsword +2, bloodlust. First attack hits for 2d6+14. the second attack misses and drains 1d6+2 from the wielder and does 1d6+2 damage to the Ogre.
The reasoning is straight-forward; the sword is mad because it likes hurting people, and your incompetence made it miss out on tasting the enemy's blood. Shame on you.
I can just see an intelligent sword talking in the middle of the battle. "Ok, idiot you better hit it the next time or you know what's gonna happen, mwahahah."