I love this dude! He is just so Terrifying and dark. And yes I do sorta wish they explained more with him. But I do more greatly feel that they handled him just fine in this episode.
Its like this.
NightMare Moon: Jealousy and Hatred
Discord: Chaos and Disharmony
Changlings/Queen: Gluttony and.....Lust(?)
King Sombra: Fear and Corruption\Darkness (my take at least).
And when people say "He had no real motivation" or "He was being evil to be evil" Thats all the reason you need. Like the Joker in the Dark Knight. He was doing all those things just cause he could. Now i am not saying that Sombra here is on the same level as Joker.
But all in all He was a good (maybe great) Villain for what they were going for. Heck he might just be the most Underrated Villain of 2012/the new year.