I love fics where Brock gleefully seduces Steve, for HYDRA and his personal entertainment, enjoying himself all the way... but I was just possessed by the desire for the opposite scenario.

Steve's clearly interested, Brock's ordered to let this play out because it'll be useful, and... he hates every single second of it. He's uncomfortable to the point of fear, hyper-aware that Steve's Winter Soldier-strong, feels trapped... He's not admitting it, but it feels like rape.

And then post-TWS, Brock gets taken prisoner by the Avengers and despite Brock's best manly efforts this gradually becomes clear. Steve already felt gross and violated, but seeing Brock fucked up about it makes it ten times worse. Brock already felt gross and violated, but seeing Steve feel guilty makes it worse because it forces him to admit he's fucked up. Basically I'd love to see that negative feedback loop. (I'd also love it if Steve's dating Sam now and is humiliated for Sam to know this about him, and tortured by doubts as to whether he'd recognize it if Sam didn't want to do something.)

tl;dr, though, Rumlow's still an asshole and an evil person who was party to Steve's rape... but he feels violated by that same act and that alone is enough to fuck Steve up about it too