The fundamental reason President Trump fired FBI Director James Comey is not because he made a patently schizophrenic statement last July about Hillary Clinton's criminal non-crime, and then usurped the Department of Justice's prosecution powers--though that was sufficient ground by itself.
The firing is about the sweetheart deal James "what-an-honest-public-servant!" Comey made with, and for, the Democrat Party presidential nominee.
Recall how long it took the FBI to get around to interviewing Mrs. Clinton. On the eve of a holiday. Without swearing her. Comey being AWOL. Her two lawyer/confederates present. No notes taken. Giving them immunity, the FBI getting nothing in return. Allowing them to recover, and eventually destroy, their computers. Etc.
That's what the brand-new Deputy Attorney, Rod Rosenstein, saw. That's why his scathing letter to the President resulted in Mr. Trump firing Comey, "immediately."