What Captain America knew of being a hero as simplistic and obstructed. But he was constructed to be the ideal and thus was granted the liberty of believing in the ideal. But this concept was false. Even the super soldier had his flaws. The ideal was an end to the threat, and the quick execution was the only sure end. Tony had taught him the urge to kill; if it could be felt it could be acted on. These Hydra terrorists would no longer be a problem, to the city or to the Avengers. They would not be allowed to spread their fear.
"I am a hero," Iron Man corrected, "and I am an Avenger." When he finally moved again, it was to contort his strict, engineered form into a looser, distinctly Tony-like posture that Tony never adopted while in the armor, making the hand on his cocked hip look alien or grotesque. Iron Man didn't appear to be aware; he flicked a hand in the air carelessly, fingers curling to catch words as he spoke; "I have some different ideas for the direction of the team. Nick was right-- the luxury of letting these guys live is a pipedream. Come on, dude, let's talk then."
His arms dropped, swinging back to rock his weight with them, starting him walking backwards away from the demolished convention center through the black ash, blank face trained on Captain America. Tony had other urges he never acted on, either. They would both learn to submit.