How does Hester's relationship to the past contribute to the meaning of the work as a whole? I put that it is both positivie and negative...

Since the greatest villain in Nathaniel Hawthorne's The Scarlet Letter may be Puritanism, the characters of Hawthorne are more symbolic than human.  As such, Roger Chillingworth represents the greyness, the steeliness of the retributive power of Puritanism.  Appearing in each scaffold scene, he serves as a reminder for Hester, the Puritan who cannot subdue her passion, that there is no escape from Puritanism's stultifying and punitive powers.  In the first scaffold scene, Chillingworth stands in the crowd outside the grey prison, a reminder from her joyless past of her having married a man whom she did not love.  In the second scaffold scene, Chillingworth looms as the foreboding presence, waiting to steal the soul of Arthur Dimmesdale--"he will be mine."  It is not until the final scaffold scene that Hester and Dimmesdale escape the now ruined, depraved man in their confessions of sin.  This is the redemptive, positive scene as they are finally free of the sin of conformity to the false codes of priniples of Puritanism.  For, they must, as Hawthorne writes,

Be true! Be true!  Show feely to the world, if not your worst, yet some trait wherby the worst may be inferred!

It is Hawthorne's indictment against the hypocrisy of a religion that demands its followers never sin.  When they do, as they must, being human, Puritanism demands that they must keep these sins secret, or be ostracized from society.  But, in such secretiveness the worm of conscience eats away at the soul, and the retributive Black Man, Chillingworth, the representative of the rigid, hypocritical religion, in its blackest hypocrisy, waits for this most vulnerable of victims.