It strikes me how horrific that is for Steve. For Steve, Bucky was the one person who always saw him. Steve lived his life being disregarded by his peers except for Bucky. Bucky was the one person who was kind to him, who stuck up for him, who knew him well enough to LIKE him. If Steve could count on something, it would be that Bucky was always there for him, always ready to be supportive. 

Steve doesn’t even comment on the fact that Bucky is a killer, he comments on the fact that Bucky doesn’t recognize him. That’s the part that’s important to Steve. His reaction isn’t “what have they done to you”, it’s “why don’t you know me”. That’s how horrific Bucky forgetting him is to Steve.