My presumptions/assumptions about Connor going into this essay:

1) Connor is one of those who doesn't easily let go of anything -- especially pain and guilt. These things have the capacity to haunt him endlessly -- and he often lets them. By staying pretty much alone in the world, he has too much time to keep dwelling on the past, on things he did, on things he might have been able to change -- but didn't. (And he says Duncan lives in the past! Pot calling the kettle black, I say.)

1a) Connor has a tendency toward single-mindedness -- when he gets an idea in his head, he has a hard time letting go of it. And, more often than not, he will talk himself into ideas and make them stronger in his mind.

2) Connor prefers being in control of a situation. He does not like to be manipulated.

3) Connor never loved a whole lot of people. After Heather's death, he constructed very thick personal armor that wasn't easy to get through. And Connor didn't really want people to get through it, either. It hurt too much when they did. And then to see them die, brutally, by some mysterious hand -- well, of course he thought he was cursed. Because he took the guilt and blame upon himself, as it is his nature to do (as we see with his mother's death). Which made him love even fewer people, which left him with more time to dwell on past guilt and pain, and no one to even try to snap him out of his brooding. Granted, it wasn't every life he touched that was destroyed -- not Heather, not Duncan up to that point, and probably others escaped the mayhem -- but enough, certainly, to make Connor write off those who escaped the curse as pure luck.

Looking at how Connor views things, it's not surprising that he came to the decision he came to -- to sacrifice himself. The 10 years in Sanctuary -- even under the drug-induced haze -- let Connor dwell far too much on his past, particularly in his guilt: over the death of his mother, who would not renounce him; over the deaths of Jacob Kell and Kell's father, murders he'd committed under extreme pain and grief, but still (in his mind) murder. (Not unlike Duncan's view of himself in FUOT.) Then the experience of having the Sanctuary annihilated around him, then finding out Kell is behind all of those deaths and Rachel's death, and realizing Kell is much stronger than he is at that point -- both physically and mentally. With all of his dwelling, Connor has lost his passion for the fight (vaguely reminiscent of Methos in his first ep), and Kell's words in the graveyard light into Connor's already nagging guilt and pain.

We don't know how many days went by between the scene in the graveyard and the rooftop fight, but I am certain Connor spent those days brooding, thinking. Coming to the conclusion that neither he nor Duncan could stop Kell alone, and that Connor did not have the will left to fight -- hence his decision that Duncan must be the one to fight Kell. And coming to the conclusion that Duncan had to have Connor's power inside him to do it.

As Connor said, he thought he'd found a "way out" with Sanctuary -- and now he'd found another "way out," through this sacrifice. And I'm sure he spent those days convincing himself more and more that this was not only the *right* thing to do, but the *only* way to accomplish what needed to be accomplished (eliminating Kell). After all, wasn't it Connor's whole goal (and what Ramirez taught him) to keep the prize way from those who would abuse it? If he couldn't do it alone, the next best thing would be to combine his power with someone else's toward the same goal. And who better, in Connor's mind, to give his power to than the only person he gave a damn about anymore, the Boy Scout whose focus on being one of the good guys was as strong as his own? To the man he considered his brother and good friend? To the only man he could really trust with his life?

By the time Connor had reached the rooftop, his mind was completely set on the idea. He was utterly resigned to it, and it showed on his face. There was nothing Duncan could have said or done by then that would have changed Connor's mind -- absolutely nothing. In spite of some things he said during their fight, Connor never wanted Duncan's head -- Connor had a couple of opportunities where he might have been able to take it, but he didn't pursue those opportunities. He wanted to push Duncan to take his head instead, to force Duncan's hand in the matter -- Connor wanted to be in control, including of his own destiny. And he was.