Yes, she was gunning for the Jewel from the start, and she was a bit deceptive in her approach, but her attribute is Generosity, not honesty.

And I don't think anyone is arguing against Rarity being selfish, cause she is. But in the end it's always clear her generosity trumps her Selfishness without much effort. I mean if Spike still kept the jewel, I doubt Rarity would hold some kind of Vendetta over it. Be a bit sad for a moment, and perhaps pull out the 'worst possible thing' line, but really there wouldn't be any hard feelings.