It's just a comic book.
But it's also a comic book with symbolic meaning and a high cultural value.
People have a right to feel however the hell they want about a thing.
People have no right to tell other people to feel or not feel a certain way about a thing.
Even though you have that right to be upset about something, that is not an excuse or intrinsic justification for hyperbole, melodrama, or other such absurd overreactive expressions.
Not everyone who feels upset is necessarily overreacting.
Not everyone who doesn't feel upset is a cold, heartless Nazi.
Reactionary thinkpiece culture is out of control.
The Manufactured Internet Outrage Machine is a serious problem.
But again, to reiterate: individuals are allowed to be upset, or outraged, or experience any other emotion about anything, ever.
They also have a right to express those feelings, in person or online.
Having or not having certain feelings about an issue does not give anyone a reason to act like a judgmental shithead.
Yes, retroactively changing established comic book continuity to make Captain America into a Nazi sleeper agent is absolutely a marketing ploy. (And yes, you fell for it.)
Just because something is a marketing plot, does not mean that is inherently good or bad.
It's also part 1 of an ongoing storyline — which is part of 75 years of the single, never-ending storyline that is Big Two comic book continuity. Which means that it's not the full story, and in fact was likely written to intentionally deceive and draw readers back in to buy the next issue, because that's what cliffhangers do.
It might even end up being a good story? Who knows? (My money's on "probably not," but YMMV)
Yes, it will inevitably be undone/erased/re-retconned out of the story.
That's neither a defense nor condemnation of this particular piece of storytelling. It's just the nature of corporate superhero comics.
Every single Captain America comic book ever written still exists, and will still exist, and can still be enjoyed in isolation, and is not nullified or ruined by this revelation.
This is all due in part to the fact that capitalism has enabled corporations to control and profit off of folklore and cultural iconography in the way that would make Hercules and Gilgamesh cry.
Perhaps most importantly: the Cosmic Cube, which was responsible for returning Steve Rogers to his youthful appearance, was already being tampered with by the Red Skull, who has previously used the Cube to alter reality and history and, oh yeah, switch bodies with Captain America, which means that Captain America was kind of already a Nazi already, and that's not even counting the time when the Red Skull lived in the cloned body of Steve Rogers for years, or any of the numerous times he infiltrated high-level positions in the U.S. government, or the fact that he currently has incomparable telepathic abilities due to being in possession of Professor X's brain and could have easily implanted a memory or something else BECAUSE COMICS.