Did the good Cap just go bad and seal the fate of the universe in 'Avengers: Endgame'?
Contains spoilers for 'Avengers: Endgame'
"The serum amplifies everything that is inside, so good becomes great; bad becomes worse. This is why you were chosen. Because the strong man who has known power all his life, may lose respect for that power, but a weak man knows the value of strength and knows... compassion." - Dr. Erskine to Steve Rogers before giving him the super serum in 'Captain America: The First Avenger.'
"Ah! Captain America! God's righteous man..."- Ultron in 'Avengers: Age of Ultron'.
"I knew it!"- Thor, when he saw Cap lifting Mjolnir in 'Avengers: Endgame'.
Not for a minute will the world buy the fact that Cap is an evil chap and there are ample instances to back that theory up. So much was the love and respect for Rogers, that when Nick Spencer wrote a comic that showed Captain America to be one of HYDRA members all along, it caused quite an uproar.
Even when Cap faked being a HYDRA agent in 'Endgame' in an attempt to retrieve Loki's scepter from Brock Rumlow and Jasper Sitwell, the line 'Hail HYDRA' was met with an uproar in theatres appreciating his quick wit. In the end battle against Thanos, he literally gave it everything he had.
But, did his actions in the final few moments of the film just make him a villain? It's a tough pill to swallow if its true, but it does make sense when we look at 'Spider-Man: Far from Home's new trailer, with Nick Fury's explanation about the decimation ripping a hole in the dimension sums up a blunder that Cap made, or rather possibly could have been brainwashed to make.
The last minutes of 'Endgame' saw Cap taking the trip to return the Infinity Stones to their right moment in time. But instead of coming back to the present, he decides to live out his life with Peggy Carter in the past thereby rupturing the time and space continuum. According to the discussion between Smart Hulk and the Ancient One, the stones if tampered with could spell catastrophe, and guess Cap just opened the door for new villains to come in and consume the earth.
Again, going by the theory that poor Cap had no idea that his actions could essentially spell doom for the universe, it's a possibility that he would do everything in his power to make it right.
But the other theory could be that Cap could have been brainwashed just like how Loki had been when he goes to return the mind stone, thereby making him choose the decision of living in the past and opening up earth to new threats by altering the time and space.
In many ways, this does seem a fair speculation because there were certain villains that Cap would be no match against - despite his new found weapon and the trusty shield. It's also entirely possible that HYDRA could have somehow found out about Cap's last mission and corrupt him. But either way, this move opens up a new chapter and hopefully, one that Marvel might answer in the days to come.