Since Truddi could not remember the abuse itself, she interpreted her guilt about the abuse as guilt that she herself had done something wrong, and came to the conclusion that she must have killed someone. At the end of the film, Truddi meets the stepfather again, but leaves him alive.
In the book, there is a confusing end sequence in which the Troops seem to visit the stepfather and proceed to kill him. Close reading will reveal that this sequence is a fairy tale of revenge being told to the children as a Christmas present, and did not actually occur.