"The Rocking Horse Winner" clearly criticises the form of absent parenting modelled by Paul's mother in this short story. It is also a diatribe against unyielding materialism and a desire for money. Both of these key themes can be argued to be responsible for Paul's eventual death, and both are largely the fault of the mother.
Paul's mother's frustrated desires and expectations for life are responsible for the repeated refrain which even the house begins to chant: "And so the house came to be haunted by the unspoken phrase: There must be more money! There must be more money!" This voice is pervasive and ever-present, and comes to prey on Paul's mind in particular.
Paul's resolution to do something about this is based on a profound love for his mother and a desire to stop these voices that haunt him so. Yet, when he gives his first winnings to his mother, her voice is "cold and distant". Although this injection of cash helps his mother afford some of the things she would want, it does not have the desired effect, for the voices in the house go "mad, like a chorus of frogs on a spring evening". The intensity of the voices and the cravings of the mother are increased the more cash Paul wins. It is this that fixates Paul on the upcoming Derby, "his blue eyes blazing with a sort of madness". Despite her misgivings, Paul's mother leaves the house for 2 days, and in her absence Paul has the most intense ride on the rocking horse and dies as a result.
Therefore it is hard to find anyone else to blame. We could argue that Basset the gardener and Uncle Oscar can share the blame, but their involvement, if at all culpable, is minimal compared to that of the mother. Even the Uncle at the end of the story is sharply critical of his sister, Paul's mother: "My God, Hester, you're eighty-odd thousand to the good, and a poor devil of a son to the bad."