Thrall isn't perfect, we've seen that.
No, that's just it, Thrall IS perfect. He can do whatever he wants, reap no consequences for it, and be the hero.
He can put Garrosh in charge, Garrosh can do all the wrong in the world, and nobody will say, "Thrall made a bad decision." or "Thrall, you made the horde into what it is now with your decision." or "Thrall you've doomed us with Garrosh!"
No. They'll say, "Garrosh is bad, and he deserves death." Nobody will blame Thrall, because he's some ultimate can do no wrong green jesus regardless that he just effectively caused the current war between the alliance and horde.
But to say that Thrall is the root of all evil in the horde would be way too speculative. Thrall appoints people that he trusts, they fall to corruption. Thrall shouldn't be held accountable for the actions of others.
Then we will have to continue to disagree, because until Thrall's head is on a pike right next to Garrosh's, nothing can be forgiven as far as I'm concerned.
Thrall is the ultimate evil because his decisions, the lack thereof, the apathy of wrangling those beneath him, and generally slappping people on the hands for mass murder is what has brought the horde full circle back to its original roots.
The Dark Horde is pretty much back, and it was Thrall's hand that guided it.
Thrall is the new Gul'dan, Garrosh is the new Blackhand. The major difference is that Thrall will continue on, while Garrosh will go the way of Blackhand.