This is a great open-ended question; therefore, the answer is up for debate and will differ from reader to reader. Which answer is defended most depends a bit on the type of reader.
A literal, scientific, and concrete answer type of reader will say that the boy died from exhaustion. David's body simply couldn't handle the pushing that he and his family were putting him through. His exhaustion left him susceptible to getting sick, and once he was sick, he never let his body rest enough to recover. He died as a consequence of this.
A reader that looks for symbolism and that "deeper meaning" might say that David is killed because of greed and materialism. His mom is greedy and materialistic, and David knows this. He wants to please his mom and earn her love. His rocking horse gives him the ability to help his mother have the stuff that she desires most. She wants material goods more than she wants a happy family. Or, she believes that having stuff is the key to a happy...
(The entire section contains 2 answers and 537 words.)