Then, explain the missiles. Why was Tony using missiles against Bucky? That's lethal enough.
Explain why he didn't use the kind of missiles that actually explode with enough force to kill everyone within a wide radius when they detonate, as opposed to the kind that I can dodge by jumping behind a car door. Explain why he didn't use the full force of the arc reactor at any other point in the fight, when it could have easily vaporized a hole right through Bucky.
The Avengers were using lethal weaponry in Civil War also, but it is also clear they are not using their full capabilities. That stuff is lethal in theory, but they all know it isn't likely to be lethal against each other due to their capabilities.
If you want to argue that Tony was using lethal weaponry against both Bucky and Cap, sure, that's literally true. But if you want to argue he was actually trying to kill them, you have to explain why he wasn't trying even 10% as hard as he could have, and using weaponry he has repeatedly demonstrated his Iron Man suits are packing for when he really wants to eliminate a threat.