Who is behind who?
Diane is against Reddington's long leash and insists that he be arrested; Cooper has to talk her into it. Red has a history, and arrangement, with Fitch. Fitch hires Garrick to hunt down Reddington. Garrick needs inside info from both Newton Philips and Diane. If Garrick or his employers could get to Newton, couldn't they get to Red in the open, without having to storm the castle, thus obviating the need for any info from either Newton or Diane?
Then, soon after this, we discover that Diane knows, and perhaps works for Fitch. Fitch by this point must know that Red was in an FBI dark site. Especially considering that the CIA (Malik) knows it too.
So hold on. Fitch hired Garrick who got insider intel via Diane. Fitch apparently doesn't know this? He chews out Diane over not having found all the moles. Does he not know it was her? And why does he care anyway if it was *his* mole?
Finally, Malik uncovers the mole is Diane, thus handing Red all the reason he needs to kill Diane, meaning a CIA agent working for the FBI just instigated the killing of a federal official (or whatever Diane is).
And then we learn that Red and Diane know each other. And Diane knows Red knows Fitch. And Red knows Diane gave Garrick (who was working for Fitch) the information to hunt Red down. (This means Diane authorized a leak that led to the death of a ton of FBI agents, too.) So... Why didn't Fitch just go directly to Diane all along? Why launch a violent attack on an FBI dark site, not to mention a few civilians, just to get to Red, when he could just pull Diane's string?
- Fitch explains this- he shows photographs to Red of him and everyone who works on the taskforce (including Diane, to throw him and the Feds off) to demonstrate that they could have killed any of them at any time they chose. They hired Garrick to attack the Blacksite to make the point to Red extra-clear- there is nowhere that he can hide from them, so he better start explaining himself. Probably the reason they used Newton was just to cover their bases, and maybe he gave them some other piece of info. That, and it helps to hide their tracks if everyone thinks the mole was Newton and has been caught. You could of course also imagine a situation where Fitch tasks Diane with setting the operation up and Diane, knowing that the thread could be traced back to her, brings in Newton solely to have a patsy even if he told them nothing they didn't already know. When Fitch chews out Diane over not finding the mole in the task force, he does so while heading a senate committee that is investigating what happened, so its easy enough to handwave that he did this for show as well and they other two people he was sitting with (whom we never see properly, but are clearly there) were not part of his conspiracy. It might be born in mind that just because Red knew Diane doesn't mean that he knew she was part of Fitch's conspiracy, or (though much less likely) he might not have known that she was the one running the taskforce.
- Basically all of this implies that the reason (or a reason) that Red has escaped justice all this time is that he has enjoyed protection from Fitch and whatever government conspiracy he is running/ is a member of, possibly in return for favours. But Red has his own agenda and turn himself into the FBI in order to pursue it. When he does this, Fitch and co. are understandably spooked as He Knows Too Much, but since he hasn't exposed them yet (to the best of their knowledge) they decided to give him a chance to explain himself- after reminding him of what they are capable of by tearing through the blacksite and his new FBI friends just to show that they can, hiring Garrick to twist the knife further since Garrick has a grudge against him. Diane, as head of Criminal Division and thus the person the taskforce ultimately answers too, is in the best position to carry this out, but to avoid tracing it back to her and to make sure that they have all the information they need, they force Newton to co-operate and frame him as the only mole. Red wasn't convinced and pursued it further, and Diane failed to properly hide her involvement, leading to her death.