The "Diablo" in the teaser could be a... Diablo Clone? Also note that the Diablo had shoulder-mouths... Weirdish.

Also, according to the back story - it was Tyrael that made the High Heavens decide to keep Sanctuary and it's mortals, and that he has been an ally to the mortals (descendants of the Nephalem (Diablo version of Nephilim)) for a very, very long time. Apparently some sort of fallen mega-angel created Sanctuary and before that there was merely the High Heavens and the Burning Hell. Which seems quite boring. Although, I guess the years Tyrael spent amongst humans might have made him susceptible to corruption...

(And as !K_A! said, you destroy the Diablo-soulstone in a video.)

"That surface that tells the Heavens it hath an end, cannot persuade me I have any."