I have played (although I no longer own) Diablo (PS1) and Diablo 2 (PC). They are both great action RPG's; however, I dislike the extent to which Diablo 2 is dominated by power gamers, pkers, and cheaters/hackers. I appreciate the complex skill system and rewarding item hunting, as well as the content that was added with the expansion (Diablo 2: Lord of Destruction).
Why don't I own Diablo 2 anymore? Funny story...I gave it to my best friend who was addicted to the game after he fried his computer. The same goes for the expansion. Unfortunately, I have a bad habit for being generous with my games (I lost SC: Brood War, Virtua Fighter 4, and F-Zero Advance in the same way
).
Whether or not the game is "demonic" depends on your definition of the word demonic. If anything featuring demons or the occult is demonic, then Diablo 2 is demonic; then again, the Bible is also demonic according to that definition. If applying the term demonic means that the game promotes occultism, demon-worship, or the ultimate triumph of evil over good, then Diablo is not demonic. As mentioned by others, the game depicts a battle between good and evil, with your character defending the good.
While Diablo 2 is not explicity Christian, virtually no game meets that criterion. Games are about entertainment, competition, commuinity, and fantasy. If they include religious elements, these are generally only of minor importance. For this reason, it is not really fair to single out Diablo 2, Doom, or any other game as a specific example of a "demonic" game that should be avoided.