I can't find a way to justify it. I haven't done it in all these years I've owned the game. I ended up cheating and got the Spec Book in Awakenings. 
What's funny is I notice many people take the Dark Ritual at the end. Which, in my view, isn't all that different. You're letting another kid get possessed... only from birth. Why that's easier, I don't know.
The justification might not have a lot of ground to stand upon, but:
1. Maybe the talents it gives you will prove vital in defeating Mr. Archie Demon. You are after all, two greenhorns and a rag tag group about to face the greatest evil in the world (at the moment.) Every help is appreciated. Maybe that blizzard spell will be the thing that will turn the tide in the final battle and you are a few talent points short of acquiring it?
2. Blood magic is the most evil of spell schools, but also seen as the most powerful. And that power might be needed to save the world. If you sacrificed Isolde you probably see the benefits of controlling such powerful magics. And if you met Avernius you might believe that your tainted blood could accomplish even greater deeds.
3. Maybe knocking boots with a desire demon is just the inspiration you need to open a can of whoop-butt on the darkspawn. You never know. The afterglow is a powerful force.
4. Maybe you are just a few approval points short of Sten strangling you in your sleep. You are a dainty mage and your neck cannot withstand such punishment. It's for the greater good.
5. Or maybe you realized that you just aren't strong enough to face the Desire Demon in combat. You ran out of health poultices in the last fight and you have no healing spells. You can't just leave the Fade to restock. The violent route will end up killing you and you are the glue that keeps the group together. Without you Alistair will have to lead and he'll end up making the group face the Archdemon without any pants. You make the deal now, get some advantage over the Archdemon and come out of this alive. It's for the greater good. (And flawed heroes are the best heroes.)
For all of these. Connor is just one poor casualty to save the rest of the world. And maybe he'll be strong enough to round house kick the demon on his own when he gets older. Maybe the Circle can help him create some defenses against the demon. It will build up his personality.
Or maybe the demon will just forget about him. Very unlikely, but still, you gotta hold onto that hope.
Maybe not the best justifications, but I feel they can work if you are roleplaying what your character was thinking. And I'm a sucker for having my characters have some sort of a dark side to them.
And interesting point on the Dark Ritual.