At the start of Warlords of Draenor, we find out Grom Hellscream is the leader of the Iron Horde and he is bent on conquering all of Draenor and any other land he can reach. In fact, most of our questing experience is seeing how the Iron Horde has assimilated the orc tribes and devastated the lives of everyone else living there, under the orders of Grom.
Then, at the end of the legendary ring chain in 6.1 we watch as Guldan seizes control of the Iron Horde and Grom is captured.
In Hellfire Citadel, we free him and he shows up in the fight against Archimonde. However, after Archimonde is defeated, we see Durotan, Yrel, and Khadgar all celebrating alongside Grom, and all their animosity seems to have vanished.
Why has everybody suddenly forgiven Grom as if the events prior to 6.2 never happened?
Best Answer
Grom redeemed himself when he refused to drink the demon blood a second time, with intent to never be enslaved. When doing so he moved to attack Gul'dan outright physically. Though he was handled easily by Gul'dan this turned him into at least opposing the same enemies as us heroes in the story. There are quite a few communities debating if he deserved this redemption story and to what degree this was glazed over by Blizzard.
Most useful 2 references: http://wowwiki.wikia.com/wiki/Grommash_Hellscream and http://www.mmo-champion.com/threads/1806309-Should-Grom-be-forgiven-(Spoilers)