Harry’s statement, and its implication, that it wasn’t Sirius who was holding him up, had resonated with Draco and stuck with him for days. It had revealed to him a fear he didn’t know he had - that he would allow Harry to fall. The Dark Lord had to be destroyed, and he had promised Harry, they would achieve it before the summer holidays, and they couldn’t do that without   all   of the Horcruxes (including the ring, a small, unhelpful, and slightly panicked voice said, but he tried to ignore it for now, as he always did). The idea that Harry might be sent back to his relatives was simply unacceptable to Draco, and he would do anything he could to prevent it.