Falling said:
I don't know. My introduction to the Warcraft universe was Warcraft II: Tides of Darkness. I always felt like I was playing the darker parts of the Silmarillion or something. I went back to Warcraft I years later and found it light on story, but not too out of keeping from the Warcraft II tone. It's really only Warcraft III and most especially WoW where the universe really gets bonkers.
As someone who's played all three Warcraft RTS games, I can't really call any of them dark. WC1 basically has no story to speak of. WC2 has a bit more narrative, but it's not a dark story, and I can only think of two actual dark moments (a point where an orc burns human corpses, and the ending of the Horde campaign in Tides of Darkness). Warcraft III has by far the darkest moments and storyline (Arthas's descent and killing Terenas, the Scourge campaign in its entirety, the feeling of dread as the Burning Legion pops up more and more through the Horde and Sentinel campaign, etc.), but is it a dark game in of itself? No, I wouldn't say so. No more than Lord of the Rings is a dark story, even when it's based on a conflict where the stakes are life and liberty for the Free Peoples of an entire continent.