What would you do if you witnessed my character kill someone?
The world seems strangely silent as the man’s body falls. They two of them - spy and inventor - hold their breath, both hoping for a hint of life, movement from the civilians’s body. The pause stretches out, until Natasha slowly turns to the man beside her, encased in a suit of armor. “Tony,” she began, and then stopped; while she had managed to beat the guilt for all the lives she had taken, that wasn’t an experience she wanted any of her comrades to share. Her mind whirled through a hundred possible things she could say; finally settled on one after a long moment. “It was an accident,” she said, quiet, and she laid a hand on the metal protecting his shoulder, facing him squarely for a moment. “I’m sorry, Tony.”
She wouldn’t wish the guilt of taking an innocent life on anyone. Let alone a friend.
Tony took the faceplate off and simply stared at the lifeless body in front of them; one thing was having to kill the 'bad guys', someone who was trying to kill you and gave you no other choice, another was killing an innocent person. Someone who had nothing to do with what they were doing there.
Now the engineer just felt dirty, if you asked him he'd tell you his heart stopped beating. "Don't." His tone was low, barely above a whisper, and he looked back at Natasha with what was a completely expressionless face. "Don't do that-- don't say it."
And then he realized that this was going to go on top of the already incredibly heavy guilt he constantly felt for all the bad things he had done; making it even heavier. A constant reminder that he was not a hero. Not one of the good guys.