Yes actually, you can become a radical inquisitor. However, more strictly to your question and a truer response; no, this game as currently planned is fighting Chaos and Xenos, not joining them. However, maybe later expansions who know.

That sounds like the developers would have to make an entire new game within a game. I love the idea of being Chaos, but I don't see it happening. Maybe in pvp mode or something, but I doubt we'll see a story line with Chaos as main.