Sort of an opposite of the overly worrying Steve thing, just like a Steve who does not realize how damaged Bucky is. He knows Hydra was horrible, he knows Bucky was tortured, he gets all of that. But like most nice, sane human beings, he doesn't realize just how sick and twisted Hydra was. So when Bucky is triggered, Steve understands that he's being reminded of horrible things but doesn't have a clue as to what it actually is.

Add to that, sexual abuse maybe just doesn't occur to him. Steve has a sort of standard patriarchal view of rape - rape doesn't really happen to men, why would a big muscular guy like Bucky be raped? etc

Then Steve finds pictures or even videos of Bucky's early conditioning and it's just horrible, horrible stuff like Bucky wearing a chain around his neck, left outside in a shed somewhere in Russia during the winter. Bucky covered in filth and hosed off, then raped and pissed on. Bucky shivering and trying to get close to his rapists for body warmth. Given dinner scraps as a 'Christmas present.' Especially videos or accounts of Bucky breaking in the early days and giving in.

+ Steve sees pictures of Bucky after he's gotten the news that Steve died and the way he's just slumped in his cell or weeping.
+Shit is so awful that Steve throws up and cries
+ Steve realizing just how far down the garbage chute goes. Never looking at Bucky the same way again.
+ All of Steve's guilt - feeling like an ass because he didn't see just how badly Bucky's been hurting.
+The kicker when Steve realizes that Bucky's actually dealing with things in a healthier way because he's going to therapy and talking about it, taking medication, etc whereas Steve has just buried all of his emotional shit.