what is it about repression that necessitates shadow build up?

I really am not sure what you mean by "shadow build up". I know that repression of all kinds exist in this Puritan community. Sexuality, individual expression, passion, sugar in their desert...you name it, if it feels good the Puritans repress it. This, of course, leads to the buildup of temptation and pretty soon they have one giant witchapalooza on their hands. Something had to break in Salem. That much repression and unhappiness lead to all kinds of nasty things going on behind closed doors.