When Barr says, "whose agents do you think you are?" to the FBI, I believe he is making a veiled legal argument. Remember that Barr believes in the "unitary executive" theory, and that therefore the president can't, e.g., obstruct justice. 1/

When he says that the FBI and DOJ prosecutors are his "agents," he is suggesting that legally they are extensions of him -- and he is also suggesting that he is an "agent" (i.e., a legal extension of) of the President. Ergo: 2/

If Trump can't obstruct justice, then nothing that Barr does on his behalf, or that he instructs his prosecutors or FBI to do, can be obstruction, either. In other words, *not only* can this entire legal arm of the government have the power and immunities of POTUS himself, but 3/

They also have no room to object, resign, whistleblow, etc. because by definition they can't be doing anything illegal or be asked to do anything illegal as long as it can be justified as being in POTUS' Art. II authority. It's a crazy argument, but I think it's what he is saying

P.S. The irony of his view of POTUS' power, of course, is that if you buy it, it means that the whole "Durham investigation" is a crock of because as long as the FBI and DOJ were acting with acquiescence from Obama, anything they do is legal. To wit: https://www.thedailybeast.com/bill-barr-cant-investigate-barack-obama-who-says-so-bill-barr