I chose to not do the Dark Ritual for a simple reason - I did not receive enough information about it.
You cannot seriously expect someone reasonable to take part in a ritual to preserve the soul of a very powerful being and then transferring it into the body of a innocent child without explaining to to them why the soul is worth preserving and what plans does Morrigan have for it.
Indeed, we learn in Inquisition that Morrigan doesn't really know what the Old God soul is. Furthermore, Old God soul Kieran is still viewed by Morrigan as a tool. After the confrontation with Flemeth in the Fade, she mentions that Kieran had a "destiny" and now its gone. Talk about being a high expectation parent. In fact, Old God Kieran reminds me of the Kryptonians from Man of Steel where they would be bred for this singular purpose instead of having the free will to choose their own destiny.
There is also the fact that Morrigan says to the Inquisitor that human Kieran's remarkable nature is due to his father and if she does well by him, it will be to inflict as little damage as possible.
Lastly, if you look at the Keep, the human Kieran is the only choice that has a bright shining light around the baby whereas a dark gloom surround the baby if conceived via the Dark Ritual.
Personally if my character romances Morrigan, I don't bring her to Redcliffe & the Denerim Alienage, I refuse to perform the Dark Ritual, get Loghain to make the killing blow on the Archdemon and then enter the Eluvian with her at Witch Hunt.