Download Captain America The First Avenger Red Skull Pictures. Infinity war prelude comic features flashbacks to that scene, and it drops a it all begins with the first avenger. In captain america firs avenger, red skull was teleported to some by space stone, i do not think that it was decided by then that he'll be guarding soul stone.
Heck, why not have it done early in the movie.
Endgame screenwriters christopher markus and stephen mcfeely revealed a load of interesting details about their massive. Captain america could've teamed up with red skull to fetch an infinity stone in avengers: Red skull reveals his true face to cap right before running away and disappointing the audience. During a fight with steve rogers, red skull did physically touch the the tesseract.