“but how could thor and the other asgardians have been completely unaware of their people’s violent history as a conquering and enslaving empire?!?” have you ever met an english person
In more detail, they aren’t completely unaware of it, they just aren’t contextualizing it in a negative way.
We know that because the audience knew long beforehand, too. It’s just that it was being presented as a good thing by the narrative.
In Thor: The Dark World, Odin flat out says that Asgard, under the rule of his father, rendered an entire race’s homeworld unlivable and consigned them to extinction. At the beginning of the first Thor movie, we find out that in the wake of the war against Jotunheim, Frost Giants are basically branded as savage monsters who had it coming to them. We never find out Jotunheim’s reasons for the war, or all the factors leading up to the conflict with Svartalfheim. It’s enough for the Asgardians present that these are just ‘savage’ people who mean harm, and Asgard is presented as having a moral duty to protect ‘the realms’ from them.
Odin frames the Jotunheim conflict as a necessary intervention to save the people of Midgard from the Frost Giants. But when Thor brings a Midgardian – a woman whose life is in grave danger, a woman who helped him in his own hour of need – to Asgard for help, Odin accuses him of disrespecting the realm by bringing such a lowly being there. Like a ‘goat at a dinner table’. A strong clue that the stated, morally upstanding reasons for the conflict are a lie – no one sends armies of soldiers to die protecting animals they’re indifferent to.
We know that Thor thinks highly of warriors and conquest. At the beginning of the first movie, he goes to Jotunheim with the intent of killing the frost giants there over the actions of a few who interrupted his coronation. He thinks this is his right, that it’s heroic and laudable behaviour. And later we discover that Thor himself is actually a pretty chill dude most of the time, and given half a chance, likes making friends and learning new things. So this isn’t one guy’s violent inclinations winning out – it’s a product of cultural values and cultural conditioning.
What’s interesting is that as his character develops, Thor decides that all this time, he’s been misunderstanding Odin’s meaning. He projects his own personal growth onto Odin – he actually learns the importance of only fighting when you need to, of defending those who can’t defend themselves, and he sees his own follies (even if that doesn’t always stop him from repeating his mistakes). It’s not an immediate process, his development into a hero doesn’t begin and end in the first movie – he’s growing slowly throughout. But he still thinks that the problem is that he misunderstood Odin’s values. That the understanding and wisdom he’s personally trying to achieve is something that his father already has.
Ragnarok is Thor learning that, in fact, no. He wasn’t just misunderstanding Odin this entire time. He was learning the lessons that Odin himself never fully understood. That all his thoughts of ‘aha, I have learned something – so Father must have already known it, and wanted me to learn it!’ were childish and naive. That’s why Odin declares, with authority, that Thor is stronger than him. Thor is better than him. Odin made monsters, he made them literally with Hela, and he made them figuratively with his propaganda about the other peoples and races of the realms. He made Asgard a realm of monsters, within and without.
Thor defeats monsters. He defeats them literally, as with Hela. And he defeats them figuratively, as when he recognizes the ‘humanity’ of the people who Odin dismissed as weaklings and villains, and sees that the ‘monsters’… aren’t. He saves what he can of Asgard, from the ruinous path that his forebears set them onto long ago.