Thrall, former Warchief of the Horde, came from a line of leaders, destined by his parentage to eventually take leadership of the Frostwolf clan -- but he ended up doing far, far more than just that. Through Thrall's efforts, the Horde was rebuilt anew into a different organization, one built not on the concept of bloody battles and victorious conquest, but instead on the ideals of freedom, liberation, unity. Family. With that goal in mind, he allowed a multitude of races to join as allies with this new Horde, establishing a home in Kalimdor.
But that Horde was not the Horde that the orc race was familiar with. It wasn't the same Horde of united clans that drank the blood of Mannoroth, ripped through the Dark Portal and stormed into Azeroth with the intent of conquering the planet. It certainly was a far, far cry from the Horde of Draenor -- the Horde from days not so long before the orcs first arrived on Azeroth, the Horde that thrived prior to Thrall's birth.
And perhaps it was that curious dichotomy between the two that led Thrall astray. Or perhaps it was simply that his parents, Durotan and Draka, were killed while he was still an infant, and he was raised in as a slave in a camp of humans that taught him human ideals, human ideas. They taught him to read, they taught him to speak, they taught him how to be. What they did not teach him was one very important thing -- how to be an orc.
Please note: The following Know Your Lore contains several spoilers for Warlords of Draenor, including the cinematics that were released last week. If you are avoiding spoilers, don't click!