Forgive my cynicism, but Barr was willing to do Trump's bidding every time he was commanded to before Trump lost the election. I honestly don't think Barr has suddenly found a line he won't cross.
Instead, I think he is engaged in calculated reputation rehabilitation. He, like the other rats, know the ship is almost sunk. He sees it as politically expedient to stand up and be virtuous in his last few days as AG.
He should be prosecuted for his crimes with the rest of them.