With the footage we have available to us what does Rey emotionally have to contend with? Why would she reach such a point of anger to strike down Kylo the way she did? What does is the reason she is so upset at Kylo during the Skype fight?
I think her anger towards Kylo comes from two places: 1) He is her primary obstacle between her and the wayfinder, which she needs in order to kill Palpatine and save her friends. 2) Kylo keeps sewing doubt in her own moral fortitude. He manifests her insecurities. So if she gets rid of him, she can erase those insecurities in a roundabout way.