Fudzie wrote...
Here's the real question: what could possibly give your character the motivation to do such a thing?
Power, pure power.
In my opinion, there should be an option of being tempted by the archdemon itself, several options should apply, one of them being that you can kill the archdemon and take its essance as you did witht he tainted blood. In doing so, you yourself become a new archdemon. (the argument of surviving it would be moot, as you have proven yourself to slay nearly all demons with ease and even carry the tainted blood in you that makes Darkspawn, if you would not survive being a archdemon, you would simply not have survived the blood drinking ritual at ostagar.
At the early part of my life as a dalish elf, i made clear i did not wanted this job as a grey warden, instead i was told i would not survive without it and thus forced. As so, I got forced in saving ferelden. The conversations with certain characters during my travels made me realise i want power, pure power. I decided to tag along and do there bidding, while my plan unfolds itself in the background, specially when talking to flemeth.
Eitherway, this option should simply be avaible, or atleast something similar to cater to us Chaotic evil characters.