After the revelation there's really no point in kidnapping Robert. Even if the queen killed Robert and Gisele, Price Edward will still be aware of her treachery.
Frankly the queen should have just gone back to her magic kingdom and close the portal. Nobody there knows what happened (or cares), she'll still reign as queen.
Maybe Narissa figured that if she kidnapped Robert and if Giselle attempted to rescue him, then Edward would make an attempt to rescue Giselle. If things worked out the way she'd planned, Edward and Giselle would both be killed off and her reign would be secure. And as for Robert? I'm sure Narissa would consider his death a bonus.
shareWith the queen, ego outweighs logic. Wasn't it obvious how Narissa relished transforming herself into a huge and powerful dragon, then whisking Robert away? The queen's overweening pride would not allow her to simply flee.
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