Steve admits to killing Tony's father, Howard Stark. Steve and Bucky join Hydra and Steve becomes Captain Hydra. He was no longer a hero...he was a villian. A enemie of the Avengers. Him and Bucky would frequently attack the Avengers and Shield. They injure many surverly. This couldn't go on forever. The Avengers excepts Tony were now actually trying to kill Steve. Tony has always been trying to kill Steve but the Avengers held him back.