Its easy enough to blame someone after all the shit happen.
Can you blame Muradin for showing Arthas Frostmourne?Or train him during youth?
Can you blame the Kirin Tor for reject Kel'thuzad experiments which eventually lead him to the Dark path?
If someone its to blame for Garrosh path its Thrall for placing the young warrior in the Highest position of one of the two most powerfull factions of Azeroth, EVEN IF EVERYBODY ELSE INCLUDING HIM SAID NO!
And yea, Varian could totally predict that Garrosh would escape in a time gate, go to the past, create a alternative timeline and a new Horde and decide to invade CURRENT timeline azeroth and lead us to the Worst expansion ever.*sarcasm*
If we should blame someone for Gul'dan, its Khadgar, we could Kill him in the spot, and still shutdown the Red portal.But no, free him, what could possibly happen if we set the most dangerous Orc in the fucking franchise history free, well we can see now what happened.