While there Diablo IV Gold have been over twenty playable characters across the Diablo series, most of them stick to the same few archetypes. Getting an entirely new character is fairly rare, and so any villainous new hero will be the exception. 

The easiest point of connection would be through Diablo's demons, particularly with Lilith so prominent in the game.

The Diablo series has spent over a decade revealing that things aren't as simple as angels = good, demons = evil. Therefore, a player character that is tied to the most reviled of these factions could make for an interesting story. 
A warlock-type character, drawing power from the Burning Hells for their own purposes would be a good way to do this. Another possible option would be a demon-summoner, binding minor demons to their service in the same way as a greater demon would.

Diablo 4 is a long way off yet, but there should be more character reveals prior to launch. Given everything known so far, it's quite possible that there'll be a morally-dubious figure among them. The looter genre is live and well. 

Destiny 2 will be ported to Xbox One and PS5, and the new hacker-slasher title Godfall aims to explore what newer, stronger consoles can do for the genre. There's another veteran looter with Diablo 4 buy Gold a new entry on the way, and even though it's not clear if it's on next-gen consoles, it's a big deal.