Who made the decision to kill the Clutters?

If a decision to kill was made, it was clearly Perry who made it; yet his account gives us reason to believe that the action was somehow automatic or unconscious, carried out while Perry was deep in a reverie of shame and self-loathing. This point is formally recognized and explored in greater depth in Perry’s psychiatric evaluation for the trial; but what is most important to note is that the robbery, which was supposed to restore the men’s dignity and self-determination, in fact makes Perry feel more helpless and contemptible than ever. Momentarily, he assigns the blame for his self-loathing to Herb Clutter, and all four Clutters pay the ultimate price for his pain.