Guilt and blame are a tricky duo in this book, lurking around every corner. Some really horrible things happen here: Estha is molested; Sophie Mol drowns; a family breaks apart. Even though the narrator sometimes suggests that these things might have been destined to happen, the only way for the characters to make sense of the tragedies they are living through is to find someone to blame. Margaret Kochamma, for instance, finds it easiest to blame Estha for Sophie Mol's death, while Chacko blames Ammu.
Along with blame, guilt is an emotion all too familiar to our characters. Unfortunately, we often see instances of guilt, or shame, where there should be none. For example, Estha feels incredibly guilty after the Orangedrink Lemondrink Man molests him, convinced that he did something wrong.
Even though it was unintentional, one could argue that Estha and Rahel are directly responsible for Sophie Mol's death.
There is no one person to blame for what happens to Sophie Mol; her death is a product of many circumstances.