Kylo Ren has taken two of the three steps needed for him to be an amazing villain. The first was his whole "tempted by the light" thing that leads to him intentionally indulging his worst self by killing Han. The second was transcending the inherent failures of the Sith by seeking to tear down galactic government instead of ruling it. The third, which will probably not happen, will be the First Order nearing victory and then getting blown up by Kylo Ren to drown the galaxy in a new age of chaos. The only reason I hold out hope for this was that I thought the second step would never actually happen after TFA but it did.
The reason the Sith are such bitches is that their ideology is as self-defeating as that of the Republic-era Jedi. The Rule of Two was meant to bring the Sith stability, but in the end it just limited their ability to grow and inevitably dead-ended in the Rule of One, which is why there are no more Sith except for p-zombie holocrons buried around the galaxy. Ultimately, the problem is that it doesn't really make any sense to use the Force to fulfill your desires. The dark side corrupts you so thoroughly you can't fulfill whatever it is you wanted in the first place, and you sit around waiting to be overthrown, just like the fucking Jedi. Going dark for something is always pointless. What Kylo Ren succeeds at in spite of everything else (and there is a lot of else about Space Nazi Boyfriend) is choosing the dark for the dark's own sake. He doesn't want to rule an empire. He doesn't want to save some girl. He's just doing what comes, and in that anti-enlightenment cannot be nullified like Sith are. The heart of darkness says kill, he kills. He meets Rey, she seems interesting, fuck it, he'll kill Snoke and roll with her. Except she doesn't want to, so fuck her too, he'll kill them all. He spends exactly zero time caring that the First Order just lost a spaceship the size of Rhode Island, kill the fucking rebels already Hux.
Kylo Ren is the inverse character of Luke. Where Luke found the way to volition within the stagnating influence of the light side that consumed the Jedi, he found the way to employ the dark by wanting for nothing and so being immune to the obsessions the dark side provokes. It's great.
And all Disney has to do is not fuck it up, which they will.
As for Rose keeping Finn from sweet death, I think what they were trying to go for was "lives aren't currency to be traded in war, that's how you become as bad as the Empire/Order, only the idealistic virtues of the rebellion set us apart" and just did a bunch of ketamine while writing that part of the story.