Thread:NerdWithAKeyboard/@comment-32039927-20180210205755/@comment-29644557-20180211013740

http://tvtropes.org/pmwiki/pmwiki.php/Monster/VideoGamesAToK


 * Al'Diabalos, better known as Diablo, is known as the Lord of Terror. Before the events of the first game, Diablo fought alongside his brothers, Mephisto and Baal, against Angiris Council in the Eternal Conflict, at times even fighting his own brothers to gain more power for himself. With the discovery of Sanctuary, Diablo sought to corrupt humanity to his will, deliberately engineering his and his brothers' own exile from hell into Sanctuary so they could corrupt humanity. Through this plan failed due to the intervention of the Horadim, Diablo's essence possessed King Leoric of Tristram 200 years after his imprisonment and later took possession of Prince Albrecht once he deemed Albrecht a suitable host. Through defeated by Albrecht's brother, Prince Aiden, Diablo corrupts Prince Aiden into the Dark Wanderer when the latter sought to contain Diablo's soulstone within himself, and procreated Leah with Adria as a means to revive himself should he be slain again. As the Dark Wanderer, Diablo destroyed Tristram with a horde of Demons and sought to free his brothers and unleash them into Sanctuary in order to corrupt the Worldstone and subsequently humanity, manipulating Marius into helping him free Baal. Through defeated and seemingly banished to the black abyss, Diablo reemerged 20 years later due to Adria's manipulations of another group of adventurers to empower the Black Soulstone and restore Diablo into Sanctuary into a form combining the power and essence of all the Great Evils. Ultimately, Adria's manipulations succeeded at restoring Diablo at the cost of Leah's life, and he ascended to the High Heavens to destroy the Angris Council, causing much destruction before his defeat at the hands of the Nephelim. Driven by a desire corrupt all of humanity and spread terror across Sanctuary, Diablo stands out as the vilest of the Prime Evils.

Additionally, Diablo does have a moral agency as he can determine when to be an ally and when to be an enemy, and knows the difference between good and evil. On top of the fact that the game makes continual clues that he was a fallen angel and was, at one point, possibly on the side of good.