Why is Azog the Defiler sworn to wipe out the line of Durin in Peter Jackson's The Hobbit movies?

Question

I've read that Azog swore an allegiance to Sauron to destroy the line of Durin. Which implies that Sauron was the one who wanted the Durin line wiped out. Like it was some part of a larger plan to regain control of Middle-earth. However I have also read that during the battle, Azog swore an oath to his forebears to wipe out the line of Durin. In both cases it would be before Thorin cuts off Azog's arm. And the movie makes it seems as if Azog is hunting Thorin out of revenge for disabling him. So why exactly is Azog sworn to wipe out the line of Durin?

I want to know the explanation for the Peter Jackson movies, not the book.

• I have more information, but still my question is not answered. In the "Desolation of Smaug" Bolg tells Azog: “They are gathering at Dol Guldur. The master has summoned you.” Azog then goes to see his master "The Necromancer" (Sauron) and The Necromancer appoints Azog to lead his armies. Azog asks “What about Oakenshield?” because it seems he's only really in it to kill Thorin who disabled him. The Necromancer replies that “War is coming,” implying that they have other priorities. Azog gets very angry and says “You promised me his head!” And even more angrily the Necromancer replies “all will die,” implying that Azog shouldn’t worry. He’ll have Thorin’s head by the end of it.

• So this new information definitely supports my idea above that Azog really only cares about killing Thorin for revenge due to his arm being cut off. But Thorin cuts off Azog's arm near the very end of the battle. And Azog began the battle sworn to wipe out the Durin line. That's why one of Azog's first acts in the battle is to behead Thror. And Thorin really only goes after Azog & cuts off his arm in retaliation.

• I have even more information. I went back to "An Unexpected Journey" and watched Balin tell Bilbo the story of the Battle at Moria. His quote is: "We went to reclaim Moria, but our enemy had gotten there first. Moria had been taken by legions of Orcs. Led by Azog the Defiler, a giant Gundabad Orc who had sworn to wipe out the line of Durin. He began by beheading the King (Thror)." So we know for a fact that Azog went into the battle already sworn to wipe the line of Durin.

So why is he sworn to wipe them out?

Answer 1

Okay, so I have some new information that I think is the final answer. I could be wrong, but with the combined information that I gave in my question and this new information I have found, I think this is it. Which is why I have decided to post an answer rather than another bullet point in my question.

Sauron's plan in the Hobbit movies basically comes down to this: Sauron’s final goal was to restore the Kingdom of Angmar and gain dominance over Middle Earth by conquering the Lonely Mountain (Erebor) because of its strategic position. Angmar was the realm of the Nazgul and often referred to as the Kingdom of Evil. It was founded for the sole purpose of the destruction of men. So by restoring Angmar, Sauron could continue his plan to wipe out the race of men. It is revealed that it was Sauron who sent the dragon Smaug, all those years ago, to take out the dwarves living in Erebor so he could use it’s strategic position to revive Angmar. But when Bard takes out the dragon and Thorin Oakenshielf & company reclaim Erebor, Sauron is forced to send his Orc army to get the Mountain back. (There might be some details out of place, but that's basically Sauron's plan).

So with this new information, I think it might confirm that it was Sauron who made his lackey Azog the Defiler to swear an oath to take out the Durin line. I mean if Sauron went through the trouble of getting a dragon to take Erebor, I think he would make sure that the rightful heirs to the Erebor throne were killed and thus could never come back to reclaim their throne. And those Orcs are basically soldiers for hire. So Sauron could have hired Azog to do this job and because he wanted it done so badly, he made Azog swear an oath to get it done. Therefore Azog would have gone into the battle at Moria sworn to wipe out the Durin line and started by beheading Thror (like Balin says in my 3rd bullet point). But then Thorin cuts off his arm and suddenly this battle and his oath has become personal. Azog now sees his oath as revenge.

I think that's it! If anyone has a better explanation, I would love to hear it!

EDIT: It's been almost a month since I posted this and no one else has offered an answer. So I'm going to go ahead & choose my own answer as the correct one. Plus I really do think it's correct and this action can be undone if my answer is proven wrong with new evidence.