First a little background to set this scene. Captain America is battling Mentallo, who has managed to connect his mind to the minds of everyone on the planet. Mentallo, decides to do some multi-tasking, and while his minions battle Cap, he sends his mind out and discovers Tony Stark is Iron Man. Mentallo than confronts Tony in the astral plane, and Tony defeats Mentallo. However, Tony decides to use Mentallo's link to everyone on the planet to erase the knowledge of his secret identity from the minds of every living being on Earth. Afterwards, Tony meets the Avengers, who were affected by his mindwipe, and reveals that Tony Stark is Iron Man. What follows is Cap and Tony having a discussion about the ethics of Tony's actions.
First things first, let me state that in theory, Tony's decision makes sense. However, if you dig beneath the surface, it becomes kind of obvious(I hope!)that Tony crossed some lines he shouldn't have crossed. Basically, Tony decided to tamper with the minds of every man, woman and child on the planet, and yet he sees absolutely nothing wrong with that. Right there, that says a lot about Tony's character, and his sheer arrogance. Yeah, Tony figures he didn't hurt anyone by erasing the knowledge that he was Iron Man, but seriously, if you were an Avenger, and you knew Tony has no qualms with fooling around inside of people's minds, could you ever trust him? Wouldn't you be suspicious of Tony from that day forward? How do you know he didn't do anything else while he was erasing his identity? Maybe he coerced some people to buy extra stock in his company, who knows. The bottom line here is that Tony felt that he had every right to play God, and is completely incapable of understanding why someone like Cap finds that wrong and reprehensible. While I can see right now that many people will take Tony's side on this issue, before you do, ask yourself honestly, do you want a billionaire invading your privacy in the most intimate way possible because he feels his actions are "noble"? I know I'd be pretty pissed if I was an Avenger and I discovered Tony was playing with my mind.