There were will be spoilers for multiple recent-ish comics. Read on at your own risk.
I'm pretty sure most of you, if not all, have seen the newest of issue of Captain America: Steve Rogers, that being the first issue. A great comic, with excellent interchanging between past and present events. However, most of the internet is incredibly angry at the bold and crazy idea that Nick Spencer (writer of the new series) has decided to take. It is revealed Cap has been a Hydra agent since the beginning.
It is explained throughout the book that the reason Cap became a Hydra agent was because his mother was saved by Hydra agent Elisa Sinclair from the abuse of his father. They went out for lunch, and as they arrived home Ms. Sinclair gave Sarah Rogers a Hydra pamphlet. The issue has Cap say "you follow your role models," which for him is the excuse to become a Hydra agent. This coming from the guy who punched Adolf Hitler in the face:
That's right folks, Captain America punched Hitler right in the face in his first ever appearance. His character was made, shaped, and defined while fighting Hydra. Now it turns out he was Hydra this whole time?! 75 years of comics and it took Marvel quite the time to tell us this. Why doesn't this add up? Let's take a look at some instances Cap could've revealed his secret or when his secret could've been revealed.
Cosmic Cubes are items known to grant users enormous power- even the power to take over the world. In this instance, Cap has it in his hands and instead uses it to restore Bucky's (as the Winter Soldier) memory. This would've been the best opportunity to, instead, take over the world and reveal his secret. Why didn't he? Some theories after the next few instances.
Oh yea people, Cap once used the Infinity Gauntlet itself. It's power has been multiple times stated as omnipotent and only 2 people in the Marvel Universe can rival or exceed it's power (the Living Tribunal and TOAA). Perhaps a bit justifiable - the universe was ending - but it's still an instance in which Cap could have made Red Skull (or himself) the ruler of the world and whatnot. One last instance that's actually made up of multiple instances:
Mjolnir is famously known for being Thor's hammer; a hammer that can be lifted by anyone who is worthy as considered by the hammer. What I want to draw out with instances like these is that a) if Cap were Hydra the entire time, Mjolnir would've known and b) Cap could've used the stinking hammer to be as powerful as Thor!
Now that all of that's out of the way, we continue onto two highly likely theories as to why Cap would utter those horrible words.
Number 1: Kobik
Not sure how many of you have read the quite recent Pleasant Hill story, in which a cosmic cube becomes a sentient/living little girl (about 6-8 years old), and is used by SHIELD to imprison supervillains, making them believe they were other people leading ordinary lives in a town called Pleasant Hill. For those who didn't know; at some point the Super Soldier Serum in Steve Rogers wore off, turning him into an old man.
That's why Sam Wilson is the current Captain America. Anyways, during the Pleasant Hill story, Baron Zemo found out and escaped the mind control-jiggy and became aware of Kobik. He tried to capture her in a device, but she was eventually rescued. While there's not signs of him programming her or anything, her restoration of his youth might just explain it. She ran away after the incident.
vv He was remade into vv
Number 2: Red Skull
There is one other theory as to why as to why Cap would begin to even think about uttering those words, and that's based all around none other than the Red Skull, Cap's main villain throughout 75 years of comics. Recently, a clone of the Red Skull found Professor Xavier's brain. Using it, he gave himself the telepathic powers of the founder of the X-Men. He clashed multiple times with the Avengers and the Unity Division (read Avengers vs. X-Men), and ultimately remade Hydra at the end. During Captain America: Steve Rogers #1, we get a lot of flashbacks from Cap. Could these be influenced in some way by the powerful new telepath in the MU? We'll have to find out as the series progresses.