From the Scarlet Letter, Ch. 10-12
Haven't read this book in years but I believe he goes back to the scaffold at night because he is being eaten away by guilt. It is a type of redemptive action, just as Hester had to stand there and be judged for her sins he feels the need to also stand there on the scaffold with Hester (theoretically) and be judged/punished as well, the only difference, of course, is that he does it under the cloak of night, when no one can see him.
