In the movie Warcraft, the Guardian is shown as corrupt, but they didn't show when he is corrupted! If he is evil from the beginning, why did he help the humans at the first encounter with orcs, in the jungle scout? Why did he ask the mage and half orc half human Garona to help the king?
My question is why did he go corrupt and why did he help the orcs to invade? Why did he change sides?
While not directly showed in the movie, it was strongly hinted that the story follows that of Warcraft lore:
Medivh (the guardian) was corrupted (or rather, possessed) even before birth by the spirit of Sargeras, the dark titan. This event happened when his mother, Aegwynn, fought and (seemingly) defeated the titan, around 800 years before the events depicted in the movie.
Sargeras' spirit remained dormant in Medivh, apparently occasionally taking control over his body to move his own plan of conquering Azeroth forward - this is why Medivh himself had no memory of letting Orcs into their world, among other things, only realising it towards the end.
His final transformation towards the end is when Sargeras seemingly takes full control over him, turning Medivh into a demon strongly resembling himself (or a dreadlord, as pointed out here).
In short, Medivh never consciously changed sides, he was merely being manipulated (possessed) by an entity far more powerful than him. Arguably, his true downfall was arrogantly dismissing this as a possibility, as he was likely aware of the possibility of it happening (shown when he burned Khadgar's drawings).
Guardian him self wants to oppose the green magic till it overcame him in the end. We can say this as when he was recovered he chooses to help humans.
He help humans at first place because he was assigned this duty. When he realize the green magic has overcome him, he asked garona to help King.
As he was living lonely, he might have attracted towards the green magic and by mistake he helped the orc. He wants to help humans but whenever the green magic overcame him, he changes side and help the orc.
The Guardian was engulfed in the Green Magic since the beginning of the movie. That gate that the Orcs go through in the beginning also needed someone on the other side to help open it. They hinted at that towards the end. Also, the half orc chick talked about she saw (well, heard his voice didnt see his face) the helper of the gate and said it was a demon which we very briefly saw the Guardian in that form also towards the end. Of course the Guardian had moments where he tried resisting the magic but was always leaving little hints of his betrayal.
