I think the battle for someone's free will goes beyond a simple good vs. evil notion. I think maybe it is an experiment to see what it is that will cause a man, (or woman) to go against thier own moral code and do something completely against their nature. For instance, what would it take to cause you or I to take an INNOCENT life? Two things come to my mind right away.....

1. Deception: you are lead to believe the innocent is a threat to you.
Examples: Anna Lucia shot Shannon believing she was one of the others coming for them. Sawyer killed an innocent man after being led to believe he was the man that destroyed his family.

2. To Protect a Loved One.
Eko killed an innocent man to save his brother. Michael shot two innocent women to save his son.

I guess the premise would be that everyone of us has a weakness, a breaking point....where we might do the unthinkable and take an innocent life. Now it seems as though Jack is square in the middle of such a crossroad. Is it coincidence.....to me it seems that it is not.