I could not agree more. The sheer amount of build up for the King was totally wasted on account of what we actually got at the end. Santa clause with grenades.....
To answer the main question though, he was definitely a Villain, I mean it was his people who were bringing about the destruction of not just the universe, but all of existence itself. I'm pretty sure that is definitely Villain behaviour, however disappointing the being known a the Crimson King himself turned out to be in the end.