The comics could explain why Red Skull was sent to guard the Soul Stone in the Avengers movies.
The Red Skull could never get Soul Stone in Avengers: Infinity War and Avengers: Endgame, and the reason why he was chosen and never got it can be understood from the comics. The obvious explanation is that he cannot sacrifice anything he loved because he is separate, but there may be more to it than just that.
Hugo Weaving/ Red Skull disappeared after interacting with a space stone at the end of Captain America: The First Avenger and for years the mystery of where he remained unresolved. In Avengers: Infinity War, he reappears on the planet Vormir – played by Ross Marquand – as Keeper of the Soul Stone.
After meeting Gamora (Zoe Saldana) and Thanos (Josh Brolin), the Red Skull set the rules for stone and explained that to get it, they had to abandon what they loved most.
For many years, the Red Skull was trapped in Vormir, unable to get one of the six Infinity Stones, even if it was to take over. In Infinity War, Red Skull explains that he will guide others to a treasure he never had.
The Red Skull cannot abandon what he loves because he is trapped on a desolate planet, but the explanation of why he can never have the Soul Stone can go much deeper than that. In Marvel Comics, the Red Skull is simply incapable of love.
In Captain America #298, as a 30-year-old drifter in Germany, Red Skull thought he loved a woman, but after she rejected his love, he angrily attacked her. Years later, he realized that it was just lust. Red Skull has never loved anyone, not even his daughter, Sin.
Red Skull’s loveless relationship with her daughter caused her to grow up to be a hateful, vengeful woman desperate for her father’s approval. As Red Skull has explained many times, his hatred of Steve Rogers is important to him. Love has never been on his mind.
The MCU has never had a chance to explore this facet of the character, but Red Skull seems to be an accurate portrayal of a red-headed, like his comic book counterpart, lacks the capacity for love. This means that Red Skull has no way of getting the Soul Stone.
If you look at the history of Red Skull’s, it seems very appropriate – and perhaps even poetic – to see Soul Stone, an object that requires someone to abandon what they love, so someone incapable of loving is never able to afford it.