I disagree, Deathwing is a tragic character. He was driven mad by the old Gods, and duped into creating the dragon soul disc, which fully corrupted him and drove him past the brink of insanity.
You're right in a way, I suppose, I guess we should think of Deathwing akin to Sauron. Sauron started out as a benevolent Maiar spirit which was seduced and eventually twisted by Morgoth. It's not so much that these characters were good to start with, but they chose a path of evil with little influence other than a nudge in that direction.
