I get what you're saying, but cap never killed Tony. Cap tried to refrain from killing at all in most circumstances. The only time we specifically hear him talk about killing someone, it was Thanos, which is completely warranted. This man was lying helpless in front of John, begging for his life, and he didn't stop. That combined with having dozens of witnesses certainly didn't help his case. Those people didn't know that guy was a terrorist. All they saw was captain America killing a man.