***Spoilers***
All but one of my Wardens romanced Morrigan, and I always carried out her ritual when my Wardens were in a romance with her.
Now, concerning Morrigan being evil (or what have you).
Morrigan is not evil, she grew up as a social isolate as a direct result of her mother's authoritarian parenting. Morrigan has trouble empathising with others. Why? Well, she was being raised to be nothing more than a vessel for her mother to hop into, once Flemeth’s own body grew too old. Flemeth wanted Morrigan magically strong, with no attachment to others, to make the transition as easy as possible. Morrigan is a very garded person, and tries not to let too many people in. If you befriend Morrigan, and or romance her, she acknowledges that much of what her mother taught her does not corresponded with her own perceptions of the world; this comes about from Morrigan’s perception of the relationship she has with The Warden. Morrigan is not a monster; she can fall in love with your Warden. She even seems a bit clingy and possessive of the Warden, (which is probably the underlying incentive behind Morrigan’s ring) it almost looks like a kind of first love syndrome. Her main dissonance (mental contradiction) comes from her mission-whatever that might be-and her love for the Warden.
In Witch Hunt, she did not have to wait for you. If you romanced Morrigan, she lets you go with her through the eluvian, even though you have been apart for two years, and she said that it was better off if you did not follow. I think her regret for leaving a happier existence, and the strain of going through child birth alone without The Warden by her side really took a toll on her (we are told at the end of DA:O that she felt regret for leaving). She also shows some guilt (if it is your Warden’s OGB) for keeping the baby and the Warden separated. Because of this, by Witch Hunt, Morrigan changed her mind about not having the Warden with her. Perhaps it was one of those cliché, you do not know what you are missing until it is gone? It seems as though plot wise, Morrigan can get her second chance with her love. She finaly lets the Warden into what ever it is she is doing, also if she truley loves the Warden I doubt she would like leaving him to a fate that ends with the Deep Roads. At least if the Warden is with her, they can be happy together for a short time. Morrigan might be a very practical person; however, much of her ‘witchy cynicism’ that she ‘learned from her mother’ was superficial, and it unravels once your Warden gets to know her.