Stuff here isn't meant to be taken seriously, I just want a place to pour the thoughts I have in my head and perhaps others are interested in reading it (as I've had requests to post stuff). Do what you will with this information.
Since the beginning of time, dragons were hunted down in their supposed homelands, and driven out of it. It was dire times for them; they needed a hero, a saviour. So their father, their god, sent them a "Dragonborn", or "first-born", ... he sent them Alduin. And Alduin proved a worthy leader, and a worthy hero. He lead his dov to safer lands that they conquered and made their home, building them an Empire were peace would rule for many years to come, in the continent of Atmora.
But all good things must come to an end. Spreading to Skyrim, the power was too much for Alduin and he became corrupt, just as past and future Dragonborns such as Talos and Miraak. The rule became tyrannical, there was misery... the humans now needed a hero, a saviour. But the damage was already done- there wasn't enough time for a human Dragonborn to be born and raised and be ready to fight off Alduin's corruption while the Dragon War currently waged. But the god of time knew that a Dragonborn would be born in the future, ready to fight for the day Miraak returned to Tamriel. For whatever reason compelled Felldir the Old to use an Elder Scroll on Alduin, the god of time cast his now-corrupt chosen child to be forwarded to the time where another chosen individual could end his reign. Where the humans who asked for help could have their plea answered.
Alduin was worshipped as a deity by the ancient Nords, but distinguished from Akatosh, much in the same way Talos was followed by his Empire and is now worshipped by them, though distinguished much more easily from Akatosh due to his exterior human form. Alduin's soul cannot be absorbed because he obtained god-hood in much the same way Talos did- while they eventually became corrupt and ruled with force and war, they both served golden eras of peace for some time to the people/dov they were meant to serve, and they were revered for the most part by their peoples. This differs to Miraak, who had no true followers, and no true rule or reign despite being Dragonborn.
Alduin didn't die, nor could he ever have been killed. Nor was he entirely evil. No Dragonborn ever truly dies in this sense, considering their soul is a direct connection to the god of time itself- "Everything mortal fades away in time, but the spirit remains. Ponder the meaning of spirit, unslaad zii. Where mortal flesh may wither and die, the spirit endures."