I don't know a whole lot of Star Wars lore, but I'm almost thinking Ren is trying to break down the dark and light sides of the Force to usher in a true unmitigated "balance," almost in a utopian socialism sort of sense. This is somewhat echoed by Rey not fighting in a purely light-side style, and I'm thinking Ren would prefer that all Force-sensitives operated based on their own merits, as opposed to attempting to wholly enforce the dichotomy of light and dark as good and evil. This is also a potential way for him to escape the repercussions of an encounter with either Snoke or Luke, were he to defect or retaliate, respectively. Maybe seeing Finn's arc will inspire him somewhat.
I can also see a twist happening where we think Ren is just trying to bring back the dark side as we assume he thought Vader was, but in reality Ren thinks he understands the "deeper meaning" being Vader's actions, or something like that.