So, for the sake of this prompt, let's say that Bucky as the Winter Soldier was still mostly Bucky personality wise, but brainwashed to obey and believe he was on the right side with Hydra.

Now he's been getting therapy, and spending a lot of time with Steve. Almost every trace of the brainwashing has vanished.

Except the Winter Soldier isn't dead. Turns out that the better Bucky feels, the farther he gets in recovering from Hydra and putting it in his past, the bigger the Winter Soldier grows under the surface. He is his own separate personality now. He is mean, he is cruel, and he is pissed. Because he never got to finish his last mission.

He decides to make his first appearance as his very own person with his own black heart right as Steve and Bucky decide to take their relationship to the next level. While they are having sex the Winter Soldier pushes Bucky aside and starts killing Steve, still thrusting into him, and telling him just how much he hates him for ruining everything and how much better things will be when he's dead.

Steve definitely doesn't have to actually die, he can be badly hurt but survive it if you want, just looking for the surprise murder attempt during sex.

Not sure if Bucky is aware of what's going on when the Winter Soldier is in control or not. You decide.