This is it. The big one. If you want to pick a primary antagonist in the Warcraft universe, look no further than Sargeras. He created the constant headache that is the demonic Burning Legion. Through various proxies and avatars he was the one who introduced orcs to the world of Azeroth in the first place. Every conflict that ensued from that point is basically his fault. And even long before that, just kicking him off of Azeroth split a continent in two.
These days, he mostly sits in the shadows. That's what happens when you're so bad that every god, dragon, and elf wants to throw you into something called "the Twisting Nether." Fallen god or not, that does not sound like a chill place for anyone to crash.
It was all basically his own fault, too. Despite leading the dark armies of hell, Sargeras isn't actually a demon himself. At least then his ex-girlfriends could say "Well, what did I expect? He's a demon. Of course he was no good for me, and also lit my entire universe on fire." But no, Sargeras is actually the furthest thing from a demon in Warcraft lore: a Titan.
Yes, the same race that elevated Deathwing to full-blown troublemaker status is also responsible for Sargeras. Not only that, they actually thought he was a great candidate to defend all life, rather than be a complete cosmic douche. Like any good deities, the Titans were very ambitious and totally clueless. But they did shape the lifeless rocks floating through the Warcraft universe into habitable planets. They've earned some leeway.
Sargeras, not so much. There were no ancient curses or whispering horrors in his case. He just fell for that old chestnut about "gazing into the abyss." For a heavenly being born before all creation, you'd think he would be immune to clichés.
After fighting demons for millennia, evil started to sound like it might be on the winning side. Given that just about every heroic character in World of Warcraft has been corrupted, cursed, or betrayed their friends to hell-spawn at one point or another, it's actually hard not to see his point. That doesn't excuse what he did next, however, which was to free all the demons he had imprisoned over the many years. He dubbed these lesser baddies the Burning Legion, and through the magic of retconning used them to kick off just about every conflict we see in World of Warcraft to this day. In a way, we have Sargeras to thank for millions of hours of productivity lost to an MMO.