It really is crazy to me how seriously everyone takes the lore in this game. It's always been B-movie shlock, even in the RTS days. I'm not saying it's unimportant, but the lore is honestly fine. It's a vessel for Blizzard to develop the game and it's never (and should not be) the forefront.
People just nitpick the crap out of it. Who honestly cares if Arthas / Sylvanas are redeemed?