Let's be fair. It was very obvious the dark wanderer in D2 was Diablo, I knew that and I only ever played about half way through the first act (many years later I played through Act2 but stopped, again). If you'd played D1 it was exactly what you expected. The hero shoves the soulstone in his head to contain it, it was terribly obvious he would be Diablo in a sequel - NPCs talked of the dark wanderer in the first act and it's like "yep, that was me in D1".
D3 story might be utter shit but let's not pretend D2 story was deep and mysterious.