His willingness to die, rather than fight, saved the galaxy- precisely where the good of attachment comes in. While Yoda and Obi-Wan told Luke to destroy Vader, Luke chose to hold out hope for his father, throwing away his weapon and proudly declaring "I am a Jedi, like my father before me". In this moment he put himself at the mercy of Vader, submitting himself into the hands of the emperor with no hope but that of Anakin's return to the light. While his attachment to Leia temporarily drove him into a moment of darkness, it was his attachment to his father then kept him from destroying him. This attachment was thrown out of balance (like the force), but when it was rebalanced (rather than extinguished), then was he able to redeem his father and save the galaxy.

But how do you reconcile the fact that his attachment to his father saved the galaxy and redeemed him to the light side? Seems to me like his love for family did more good than harm.