Doctor Glocktor said:
Thrall met Tarethra, who showed him humans can be good. THAT is why he can trust the Alliance; he actually had a good human show him that the human race isn't evil. Who has Varian had, hmmm? Garona murdered his father, Thrall lets the Warsong and the Forsaken actively murder members of the Alliance, and Garrosh is Garrosh.
He also met Thrall. Not many people know about Garona lore wise, afterall they were utterly alone and most of Llanes people knew nothing over her. Remeber she hid in a curtain during meetings. Thats how she could so easily kill him. He has hundreds of allies that interact and work with the horde every day who basically wave a huge sign saying "hey dudes! These guys are cool! Leave me alone!"
The Forsaken don't murder the Alliance, they fight the scarlet crusade. They aren't part of the alliance actually. There level of alliance murder is right on par with alliance horde murder.
Actually, the warsong conflict was mostly in game. Lore wise, the horde and the night elves of ashenvale traded, quite often, up until the northrend expedition took so many resources that they honestly couldn't, as well as the Wrathgate where Varynn lost his shit. They make note of this in the shattering.
Also, take note, WARCRAFT 3. You know, where if it wasn't for Thrall leading the horde across the sea in the first place the world would have been fucked. MANY humans worked side by side with the horde. You would think Varynn would listen to some of those guys... Apparently not.
THE ARGENT FUCKING DAWN
I've read the Shattering as well, and you're thinking of the wrong sceneario. Only Cairne and Thrall knew of the meeting of the night elves and tauren. The only one that Varian heard about is the one where the night elves were cut up and had their skins painted with a Horde emblem. And remember; you, as a player have the knowledge that it was the Twilights Hammer. Varian did not.
It's Varynn's paranoia and eagerness to damn the horde.
Remember, the horde has NEVER done something like that before. Thrall kinda points this out.
Jania is repeating like a Parrot that it wasn't the horde.
Thrall responded to Varynn about the incident, promising to punish those responsible. Of course he didn't apologize, his people were starving because of the no trading. It would look weak in a time of civil unrest.
Varynn makes no attempt to investigate the matter. He simply screams "THEM EVIL HORDE! THEY DO IT RAWR!" without thinking, for a second, "hey, even if it was honestly members of the horde what if they did it outside of the hordes knowledge! Sorta Like the Defias?"
He INSTANTLY jumped to the conclusion they did it under direct orders from Thrall.
Despite the fact the Horde were slaughtered at the Wrathgate, there were still humans locked
up for experiments in the undercity,
Scarlet Cruesade
and the fact that Thrall is such a bad leader he wouldn't know about the plague, or so afraid of Sylvanas that he wouldn't tell her to cut that shit out.
Sylvanas hid the plauge because she knew Thrall would have the foresight to stop her. However Thrall made the
terrible mistake of trusting his allies rather then constantly investigating the inner most deepest parts of there sanctums(Remember, most of the undercity didn't even know about the plague)
That's sorta like saying the only reason Moria came to power is because Varynn was to shitty of a leader to make sure Muradin wasn't an asshole to his daughter and have her husband murdered.
He obviously isn't afraid seeing as how as soon as the incident was over Undercity is now crawling with Korkron. With one of it's leader specifically in the laboratories.
Andiun is a good kid, but Varian is the best leader of the Alliance because hes the only one who FIGHTS BACK.
Yes fight back agaisnt...
The orcs who moved to a desert called the BARRENS to try and get as far away from everyone as they could and only find conflict in the navy who followed them to the desert.
The trolls who just want there fucking echo isles and to be left alone.
The Tauren of mulgore who, once again, just want to be left alone to there eternal hunt and up till now Varynn didn't know fuck all about them.
The Forsaken that only wanted two things. Lorderan, Arthases head on a fucking platter.
The blood elves the humans basically said "Go die in a fucking hole" to. For no reason past "Man fuck those guys"
And the goblins, whom only ended up allying with the horde because the alliance tried to murder Thrall and his girlfriend as they were off to another dastardly deed... No wait, they were off to save the world.