You are one of the definitions of not knowing his character. Deathwing's motivations are far from his will, he is a puppet corrupted by the old gods and twisted to do their bidding. By bringing azeroth's destruction he sets the old gods free to kill or corrupt the sleeping titan inside of the planet, which is the ultimate plan of the void lords that sent those old gods to azeroth.
If you even think a bit, in one of the caverns of time dungeons you can see the "End of Time", where it shows Deathwing's plans complete. There he lies as a tossed away tool in an empty husk, dead. He is a tool, nothing more, nothing less.
Deathwing's character deteriorated because the more he was controlled the more mindless he seemed.
Was the way they introduced his raid encounter good? People expected a nefarian-like fight, but we didnt get one, that's why people were disappointed. Well, its kind of hard to present a combat against a dragon stronger than all of the other aspects put together as something feasible for random joes. Truth in all, Deathwing is probably the strongest villan ever in world of warcraft history, except maybe archimonde and kil'jaeden, that we had the chance to fight. Measured against old gods is tricky, since I believe they are just hard to deal with instead of strong, but I can live with them also being stronger.