I think if Morgan played Prince Charming, it would be too frightening or not believable to the UnSub.

Unfortunately, given the Eurocentrism of children's fairytale books in the U.S., that might well be so. More to the point, all her victims were fairly young white men; it was Reid who therefore fit her ideal/victim profile (and who, moreover, has shown an exceptional knack for getting into the heads of delusional UNSUBS; see "Derailed" and "Uncanny Valley". to name a couple.)