Every villain in the Warcraft universe has a story behind why exactly they're a villain. For some, it was the corruption of the Old Gods that slowly drove them mad. For others, it was the promise of great power by agents of the Burning Legion. But it's rare that we see a villain that is simply a villain without any kind of outside influence. Garrosh Hellscream is one of the better villains in Warcraft solely because he does evil things, but doesn't believe that they are evil -- instead, he believes that they are simply the right thing to do.
But before Garrosh Hellscream was an orc who pretty much represented pure, unmitigated evil in its most concentrated form. He knew the corruption of his entire race was at stake, and he went ahead and traded it for the one thing he craved above all else -- power. Gul'dan may not have come from bloodlines that boasted glory, but he was ridiculously intelligent, cunning, and cruel. To Gul'dan, it wasn't about performing evil deeds. It was about amassing as much power and prestige as he could -- and the rest of the world could burn, for all he cared.