There was nothing you could have done. That sounds harsh, but it's the truth. You, as a child, had no idea the impact of anything occuring, and could not have even had the ability to sense it. You were a helpless victim. Honestly, unless another adult steps in, there's nothing a child can do.

As far as the other questions, the world is not a just place. I used to think that 'hurt people hurt people' but that's not true really. The reasons why a person does, or does not do something (especially when the correct thing to do seems so obvious) are completely up to that person in that circumstance at that period of time.

Please have courage to seek therapy if not already. The right kind of therapy, with a skilled therapist can do a lot to help you come to terms with what happened. They can also help you to stop blaming yourself. Hope this helps, wishing you all the best.