Reddington is dying...
This is another theory I have about The Blacklist.
Raymond 'Red' Reddington is dying and that is what all his closest people keep telling him 'Elizabeth deserves to know the truth' about.
The first hint about this is in SE1, EP8 when Elizabeth's foster father calls her from his hospital bed. Sam Milhoan has terminal cancer. He tells her he is in just having a few tests.
A nurse tending to Sam overhears his conversation with Elizabeth and tells him he should tell Elizabeth - that she 'deserves to know'. Later in the episode Sam tells Raymond he only has six weeks to live and that he wants to tell Lizzie Red's secret, Raymond says, 'No'. Sam responds with, 'She deserves to know the truth'.
Raymond smothers Sam with a pillow, though it is implied Sam wants to die and that Raymond is doing him a kindness. Sam thanks him beforehand. Raymond is distressed while smothering Sam and kisses him goodbye before leaving.
In following seasons we see Reddington using medications, injecting himsel, and drinking green smoothies. At one point he involves the task force in a case about a doctor willing to trial experimental cures on terminal patients desperate to try anything. At the end of that episode Raymond confirms that this doctor is still working on a case for him.
How does Reddington surrendering to the FBI fit in with this situation of him dying? Well, partly it affords him the opportunity to be close to his daughter, who he has had to watch from afar throughout her life. Additionally, it allows him to eliminate threats to Elizabeth's safety after he dies.
The Blacklist is Red's bucket list.
He is having a bet each way, if he is able to secure a cure for the illness he is dying from, great! If not, he wants to do what he can to protect Lizzie after his death.
...an additional twist would be that what Red has is congenital and there is a chance he might have passed whatever it is to Lizzie - so, the cure might be for her.
I'm about halfway through the first season again. There may be further theories coming.