To me, the one who killed Adeline is her parents. They are the one who had indirectly killed Adeline. High expectations and comparisons has led adeline to depression and therefore cannot focus on her work. However, the parents did not notice it and add on to her pressure. I think that it is fine to compare the results of their daughter to others. However, they should not compare it infront of their child as the child might not be able to take it. The child may stress herself up to study harder to satisfy the parents as the result.
Next, the parent has also neglected that Adeline may not be able to relieve her stress and made some remarks which demoralize her. "I will die if you do not get a good grade" is one of the remarks made by the mother. They should try to inderstand the child's plight and encourage her. Knowing the child's standard and setting the right goal is also important. However, they did not, which cause Adeline's death.
We should not blame the teacher or principal as it is the right thing to be firm and strict about the marking system. If they were lenient and gave the one mark to Adeline, pupils may mistaken and ask for more marks and it will never be like an exam anymore.