Barbara Hershey does an amazing job here but the movie mis-portrayed several facts from the book:
1. It showed Peggy Blankenship as physically much larger than Candy Morrison and therefore more threatening. The reverse is actually true. This substantially undercuts Candy's claim of self defense.
2. It showed the killing happening in late winter/early spring. It actually took place in June. This is important because Candy was wearing flip-flops she later destroyed to hide them from the police. This was left out completely.