I've always thought that the Sith were vastly outnumbered, so him killing the Jedis brought balance between Dark/Light. That's the closest thing he's done to balance something. That's my way to see it. But I think that the official would be that he defeated the Emperor and that brought balance to the Force. I don't know why killing all the Sith would bring balance, that's not balance at all XD. I don't really know what it means tbh.
I don't read too much into Star Wars plots and storyline, since I think it's all about world and characters, and that's one of the things Lucas failed to understand in the prequels, writing bad characters and focusing too much on the main plot, which was messy too XD. And that something they really managed to acknowledge in TFA. And I was really happy about that!
