He's established as a willful servant of Gul'dan, just like everyone else. Gul'Dan is just as evil at the beginning of the movie as he is at the end of it, so it should be pretty obvious to everyone that he isn't a very honourable orc, you know, with all the lifesucking and stuff. Then, after invading an innocent world - by means of genocide of the Draenei - he suddenly starts to realize: "Hey! Wait a second! Maybe Gul'dan is kinda evil? Huh!". There's no real reason for this. He sees Blackhand rounding up humans and seems to be bothered by it (why? You KNEW this would happen, it was the entire purpose of the invasion and you already did it with the Draenei) and actually has to thank Gul'Dan for saving his son's life. As far as he knows Gul'dan did a pretty cool thing with the reanimation and, other than that, did EXACTLY what he promised to do.
Still, Durotan suddenly decides that he has to be a main character in this movie and rebel against Gul'Dan only because the scripts tells him to, not because he went through a meaningful character arc. They try to excuse this by him saying "Hey, notice how this lands dies? Maybe it's Gul'Dan!" to which I say "Well, bullshit!" because there is no way in hell they didn't recognize this happening with Draenor. I mean, the second Gul'Dan rose to power an entire planet died (at least that's implied? The whole Draenor plotline is painfully underdeveloped in the movie) and he just kinda shrugged it off, but the second they about 20 square meters of land turns bad, he gets suspicious and immediatly tries to rebel? Eh.
So Durotan decides to rebel by working with the humans. This alliance is what the entire marketing built toward and what this movie was supposed to show me. He meets with the humans...and immediatly gets ambushed and thrown into jail, with his whole clan getting slaughtered. That's supposed to be really shocking but since we have not even once seen the "clan" they keep talking about beforehand and gave us absolutely no information on what "clan" means in this context, how big it is or how Durotan really leads it (they told us but not once have they shown us, something this movie loves to do) I didn't care about the Frostwolves at all.
Anyway, Durotan get's thrown into jail, gets freed by Orgrim (who, again, decides that Gul'dan really IS evil after seeing him do exactly what he did before and what he promised to do - he also HAD to have known what would happen to Durotans clan when he betrayed him) and then they finally decide to start the rebellion.
Durotan fights Gul'Dan and...gets fucking destroyed. Orgrim uses that chance to talk to about Gul'Dans lack of honour and why he isn't a fit leader of the clan, three orcs kinda agree with him, get killed...and everyone else just shrugs it off and start fighting the humans, while Orgrim just continues to stand there for the entirety of the battle.
Durotan did NOTHING of consequence in this movie. He failed at everything he tried to do and was a willful participant in Gul'Dans blatantly evil schemes until he wasn't...not because there was a big event or because of character development, but simply because the script said so. He is the character the movie spends the most time with and he is completely and utterly worthless. Nothing would have changed if you would just delete him from the movie completely (well, his clan would still be alive, so there's that).
His only purpose was to be the father of Thrall, which happened before the movie and, again, has absolutely no consequence in this movie at all, since it's just set-up for a sequel. I was utterly dumbfounded when Durotan just kinda died without consequence at the end because it's such a non-existant character arc.
And still, he will probably be heralded as a hero in the sequels, just because that's what his character was supposed to be according to the plot outline, even though nothing in the actualy movie supports this view. If we just look at what he actually did he should be immediatly forgotten and shunned because he first participated in Gul'Dan's plans, then decided his plans were bad waaaay too late and then immediatly failed to stop them.
