In it, character A and B are arguing. It gets ugly, and then there is something along the lines of "A raises his hand. Before he even realizes what he's doing, his palm connects with B's cheek."
A is horrified at what he did. I think the fic might have had dub/noncon in it, but I'm not sure about that. It also might have been a dom/sub or alpha/omega AU, with A as the dom or alpha, but again I'm not sure. The thing I specifically remembers is it's from A's perspective, and hitting B is like an out of body experience.
I think A was Steve and B was Tony, but it might have been Phil/Clint or Steve/Bucky.
I wish I could provide more concrete info, but my memory of the fic is very fuzzy. I will be very impressed if anyone recalls this fic, but I figured I'd give it a shot! Thanks <3