It worked for me because, despite her upbringing, Rey is very much an innocent idealist. But also, her other attachments are: Leia, BB8, Han, chewie, Finn and Luke. Th first two, of course, are pure good. The next 2 are career criminals who became heroes and then career criminals again who became her dads. Then we have the strongest attachment, a first order stormtrooper who DID change, followed by a grumpy old guy she'd already spent the whole movie trying to get through to. It made perfect sense to me that she'd jump at the chance to "save" Kylo with all that her her frustration with Luke.