Anakin/Vader was great because we discovered his layers along the way; Kylo is the direct opposite in that we've already seen his layers, but getting the layers and then stopping there and having less development across two more movies which could be used to make him even more interesting would be kind of a bummer I think. I'm not saying you can't make a purely evil dickbag interesting and further developed, but it's tricky to do so while keeping the audience at least a bit sympathetic.
I want to expand on that before I sound crazy. Kylo was pretty bad in VII, but it was written to where you feel like he has personal reasons and demons for his actions, much like Anakin, so he isn't this 100% despicable character. He was clearly conflicted through the entire thing; hell I'd argue he was still conflicted during the saber duel. I think he could've killed Finn had he really wanted and that slice up the back seemed very calculated. Kylo was super pissed at him for his defection, yet still didn't just immediately kill him. And he could have if he wanted. So while he had just committed a murder and apparently in turn committed to the dark side, I don't think he was fully over there.
You may disagree with this and that's totally great as we're all speculating, but while I'm pissed at Kylo for what he did, when I think about it and take certain things like that into consideration, I don't think they're going to make it that simple. He did something terrible and then injured Finn, but... again, only injured Finn. It's possible that he thought he had killed him, but I don't know. After that defection you'd think Kylo would have enough justification to cut his fucking head off on the spot, yet he toyed with him for a bit and didn't go all-in. This tells me something. I realize he was also injured by the bow blast, but if he was really that upset with Finn and really wanted to kill him, I think it would've happened on the spot.
I just think Kylo was a fantastic character because there was more to him and I don't know that I would love it if he was just evil going forward without much else to say about it. We could learn more about his past that would help add more, but I really hope they find a way to keep him as interesting as he was in this one. That's pretty much all I ask, regardless of the direction they go in and I think they're good enough storytellers to keep it compelling so here's hoping. Will he get darker and more evil? Probably, so I'm not totally disagreeing, I'm just saying I get more out of the character than "I'm super evil now" and I think there's gonna be more to it.