Yoda was explicit that there is a Light Side and a Dark side period and Star Wars gets these dualities from the founding myths and legends it draws on fir inspiration. Part of the escapism of Star Wars too is the clear cut good and evil and the defeat if the latter by the former.
Kylo Ren was defeated because he was shot by Chewbacca in the side with a bowcaster and was mentally and emotionally unhinged and unstable from just killing his father which by the way is NOT. something a "Gray Jedi " if there is such a thing would do but ONLY a Dark Jedi!
Rey got so good so fast IMO because like Kylo she was a trained apprentice at the Jedi Academy and when Kylo came back with The Knights of Ren to wipe his classmates out Luke rescued her and took her to Jakku for safe keeping and used the Force to wipe her memory. When she touched Luke's lightsaber in Nas's treasure room it broke the seal on her amnesia allowing her memories and later force abilities to come back. That is why Ren was like "What Girl!?" Also he was telling that female stormtrooper "the longer she is loose the more dabgerous she becomes!"