There aren't many sane people who believe Trump's explanation for his decision to fire the FBI Director. This article describes some of the events that led to his rash decision. Some Republicans had urged him to fire Comey because he chose not to prosecute Clinton for her use of a private email server. Trump had been concerned that the FBI gave preference to the Russian investigation over the investigation of leaks that worried him. Most of the leaks were not about classified information. Its the kind of thing that happens in Washington every day. Reporters typically use sources that they have cultivated to learn more about issues on which the report. Kellyanne Conway, gave the case away when she attempted to defend Trump on CNN. She accused CNN of saying Russia, Russia, all of the time instead of reporting on leaks. She clearly understood what bothered her boss.
Trump's behavior confirmed much about what we already know about his temperament and his need for constant approval. But we also learned a lot about his management style. He tried to divert attention from his role in the evaluation of Comey. He turned that over to his Attorney General and the Assistant AG who was asked to evaluate Comey's handling of the Clinton investigation. The AG provided a criticism that lined up pretty much like the critique that Democrats made. The Assistant AG was surprised when that was used to justify firing Comey. Trump and Sessions expected that this explanation would prevent criticism from Democrats. That was really stupid. However, it was not what really worried Trump. Comey had requested more resources to pursue the Russian investigation and he told a TV audience that the Trump campaign was a target for collusion. Associates of Michael Flynn had also been told to that they were under investigation.