Marvel really needs to watch it with this "Hail Hydra" thing. It's the cheapest and easiest possible twist now to have a character say it, and there's a point where it becomes very offensive. In Age of Ultron, they didn't get into Wanda and Pietro's Jewishness (often depicted as children of Magneto, as recently as FOX's X-Men films). Then they made them agents of Hydra, depicted clearly in the films as a high-tech Nazi organization too extreme for even Hitler. If you're gonna keep trotting out this stuff as cartoon villainy, you have to acknowledge the very real history behind it, and get how culturally insensitive this is.
(Think of what "Hail Hydra" is supposed to sound like.)
Comics fans are saying this won't have any effect on the character in the long term, but sometimes these things do. Look at what happened to Hank Pym, when his character went dark and never recovered from it. (Resulting in the movies going with an entirely different Ant-Man and Wasp, fridging Janet for Hank's actions.) Sometimes these storylines have consequences. It's shocking-for-the-sake-of-it clickbait either way. We'll have to see.
Beyond the very possible offensiveness of the Nazism, there's something here which is offending people by making them feel old, that the Captain America they knew as kids doesn't matter. It's impossible for the 60s, 70s and 80s Captain America to have been a Hydra Agent all along, but it's entirely possible for a Captain America who's only been around for a few years (or one season of a TV show like Agents of SHIELD).
Remember the classic Simpsons episode (or episodes) with young Homer and Marge in the 70s? Then a few years later they showed young Homer and Marge in the 90s, making all sorts of stereotypical 90s jokes. It was offputting because it's not the characters we knew, and in a lightning bolt shows that decades have now passed and somewhere underneath that history has had to be wiped away. That's always happening with these characters so it has to be handled with subtlety, and retain the core of the character as much as possible. It's always been a little tricky with Cap and Nick Fury and Black Widow and so on, who are so rooted in the past.
Chris Evans has made Captain America much more popular than ever, so all we'd be waiting for is for someone to take Evil!Cap down and get back to some kind of normal.
Thanks to the movies, Marvel's characters are hugely famous in forms that are generally very recognizable from the comics. But in the comics themselves right now, Marvel is being more experimental. Trying out characters who haven't gotten as much time in the limelight. A lot of this is great. But this time they might really have broken something.