Characters in Light in August are distinguished from one another through how they perceive their free will. Christmas struggles with these concepts throughout the book, constantly referring to warring factions inside of himself – the black and the white, the violent person and the non-violent, the child and the man – and usually blaming others for his problems rather than admitting to his own choice in the situation. Hightower struggles with these things too, but eventually he comes to accept that he made certain decisions in life (he chose to ignore his wife, for example) which had certain effects (his wife cheated on him and died). Christmas is never able to take responsibility for killing Miss Burden because he believes he was compelled by something totally out of his control.
The idea of fate allows Christmas to shirk responsibility for killing Miss Burden.
Men in the novel are far freer than women.