That's not really how it was though. She didn't make a false promise. They made an arrangement. Even once they returned, Nicole had the doubts about whether it would work out, particularly given Sid's initial reaction, and Marilyn kept insisting it's fine. The longer that carried on, the longer Nicole probably felt quite uncertain, yet she was still the one that was carrying the baby.
As for Marilyn deserving the baby more, may I ask why you think that? Is it because Marilyn's sadly lost a child before? That's hardly a reason for somebody to "deserve" a child more than somebody else. Nicole can't help the fact Marilyn has been through that.
As for Nicole never wanting the baby, that's not entirely true. She didn't want the baby to begin with (and I agree with Miranda, that may well have been just the initital reaction to the out-of-the-blue, unexpected, shock turn her life took with pretty much no warning.) but, at the very least, that may have only been a few weeks. We don't know exactly how long before Nicole made the confession to Angelo she realised herself she wanted to keep it. It may have been that she'd known for a while she felt something, but she didn't want to say anything until she knew for certain how she felt (and she sounded pretty certain when she told Angelo.)
I understand Nicole's shock in the first time and how she try to handle it was really great entertaining, but than she was seeing how Marilyn was in love with her baby and she was doing everything for it. Marilyn told her so many times, that it's her last time to get an baby. I know she can try a real adoption, but Nicole know how she feel and hurt her so much.
And yeah it's because she lost a child and because she is a mother. Little bit childish but such a lovely woman and my biggest wish is a child for her. I know it's just an soap opera and not real, but I feel so sorry for Marilyn. And Nicole isn't that ready for an baby I think. I don't want to say that she isn't a good mother but she is not ready.