Tony was a little stumped, but he wasn't in a hurry.
Steve and Bucky had already fallen, and Luke clearly had no intention of letting them go, so waiting for a bit wasn't a big deal.
After taking care of Steve, Luke took out a military knife from his inventory and placed it in Steve's hand.
Then, he grabbed Steve's hand and stabbed Bucky in the chest.
Steve: "…"
Tony: "…"
Luke got up and threw out a rope dart to tie Steve up. He then looked at Tony and said, "You can consider this as Steve doing the stabbing, which cancels out the fact that he lied to you."
Steve: "…"
Tony: "…"
They understood what Knight was doing.
If it was Bucky's fault that he had killed Tony's parents under someone else's control, then it was indeed Steve's fault that he had stabbed Bucky under Luke's control.
But… they had both seen with their own eyes Luke grab Steve's hand to stab Bucky, which wasn't right.
