Blizzard's writers have a standard way of making new bad guys: take some good guys and make them go insane. Violin: bad guys. This method is criticized for being lazy and formulaic. I disagree. The nature of the Warcraft universe requires that any true enemies be either innately evil or have been corrupted/gone insane.
Look at it in context. Within the real world greed is a powerful driver. It pushes people to do stupid or evil things. But how powerful can greed really be, in the Warcraft universe? In the real world, greed can get you killed. That's nothing in Warcraft. In that universe, greed can get you tortured for all eternity, and I don't mean "I read in a book that if I am greedy I'll get tortured for all eternity", I mean that you have literally seen demons and magic and know quite well that eternal torture is a strong possibility.
When there are forces that seek to unmake reality itself, everyone is on the same side. This idea first came to me from the Alvin Maker series by Orson Scott Card, my number one writer for veiled homophobic writing. In the series the hero is in a fight against the Unmaker, a being who seeks to unmake everything, somewhat reminiscent of the Burning Legion. A wise old man tells Alvin that even Satan, the obvious enemy of God, would on his side in this, because Satan wants to rule something. Even evil hates chaos.
From this perspective, we can see how it makes no sense at all to ally with the Burning Legion. There is no reason to trust the demons. Given their propensity for cruelty, it is not hard to imagine them turning on all their allies and destroying them as well. Any alliance is merely putting off the day of personal horribleness while increasing the chances of it coming. It would be like guaranteeing you won't get cancer today by taking a drug that doubles you risk of cancer tomorrow.
One cannot ally with chaos, one can only join chaos. Think of when you see a protest. The protestors are thinking of a cause, a reason. You may disagree with them, but odds are, they are sane. Now contrast that with a riot, of broken windows and arson and police being attacked. Are those people thinking or considering the costs and benefits of throwing a rock through a window? No. They are chaotic. They are, temporarily, uncontrolled, inhuman, and insane.
Think of Kael'thas, who had once sacrificed everything to protect his people, and who then joined the Legion. It could be nothing less than corruption of his very nature, insanity. There is no future for his people in the Legion. Similarly, the Lich King could not be just an angry prince who went a bit too far. He had to be corrupted because only that would allow him to turn against his father, kingdom, and the very world. Merely being a little less empathetic and a little more fanatical would not do the trick. The Scarlet Crusade is another group which is not merely extremist, but entirely out-of-touch with reality. They would not otherwise attack anyone on sight as Scourge (note that I did not say "possible" or "suspected").
Beside the enemies, think of our own actions. When the world is at risk of ceasing to exist, are we going to quibble over small matters? We may argue strategy or tactics, but when there is a demonic army coming, it's not so important whether there is a troll standing over there. We're not going to go raid Stormwind just because Varian talked some smack while Deathwing is cataclysmizing the world.
On the other hand, there is also the "really stupid, short-minded idiot" method, of making someone a threat to the world not because they are evil or in league with evil, but because their sheer stupidity is threatening survival: for example, Garrosh and other orcs who kept picking fights during the campaign in Icecrown. I wouldn't call them evil, but we'd definitely have been better off killing them before they could do any more damage.
Either way, we're not likely to be flying off to a raid against a reasonable person with whom we have a legitimate disagreement.