Essentially, Light in August, is about a patriarchal society where women and Black people are marginalized. A harsh Christian outlook, judgemental and austere rather than compassionate and forgiving, dominates the characters. Misunderstanding and misinterpretations abound as there can be only one route to righteousness. This seems to forgive all cruelties and crimes committed by the men, as they ultimately believe it's for the (or their) greater good.
Justice is therefore biased and metered out in terms of the Puritanical society in which the characters exist. Speculation and assumption are valued concepts in Jefferson and considered truths, thus preventing any kind of normal society.
Joe Christmas is so severely displaced that he cannot accept, a place in any society and his violence towards others stems from his self-hatred and lack of identity. As wrong as his actions are, they are mitigated by his circumstances. His character is not however fully developed and his heinous actions and murder of Joanna do not justify his rationale in punishing (and ultimately murdering) her.
Christmas's own murder is vicious and vengeful as "pent black blood seemed to rush like a released breath. It seemed to rush out of his pale body like the rush of sparks from a rising rocket." Still there is no real justice. Hightower's realization comes too late and his provision of an alibi is unheeded. "Poor man. Poor mankind" is apparently enough to redeem him but is ineffectual.
The women are victims and even small acts of kindness are ignored because of their supposed evil ways. There is a sexuality in the women that is largely misunderstood. Bobbie Allen's "inner corruption of the spirit" is enough to ensure that she and Joe have no future. Joanna's "burden" and apparent love for Black people is frowned upon. Joe, ironically, becomes her undoing. Lena, despite having a child out of wedlock, does become the saving grace for the women and she is symbolic, a woman with "eyes that a man could not have lied to if he had wanted."
In terms of a strict, Christian belief, the concept of atonement is easy to understand. Even the criminals as they were crucified with Jesus, obtained forgiveness if they accepted God, no matter what terrible things they had done before.
Forgiveness should be unconditional when the perpetrator is truly sorry but a subjective version of religion which punishes people for their differences, as if, as in this case, Jefferson's way is the right way is dangerous and disastrous. This conflicts with Christian principles of kindness to others and "do unto others what you would have them do unto you." No one is entitled to "cast the first stone" unless they are themselves free from guilt - not the case in Light in August.