"Which would mean that Holder knew that somebody who was under his charge planned and executed this atrocity that resulted in the deaths of at least 200 people."

No which would mean that someone under his very indirect charge confessed to "walking guns."

You blame the agent who facilitated the sale but ignore the gun makers who lobbied for it and the Arizona Legislature that made the unrecorded bulk automatic weapons sales legal? Could your motivations and arguments be political? :-p