As for reasons to fire Comey, there were plenty — but the one stated by the President, seems pretty straight forward and justifiable: he wasn’t able to lead the FBI effectively.
Except that was not the reason actually stated by the President. That was ther reason given by various White House officials, but Trump refused to play along. The reason stated by the President was because “I faced great pressure because of Russia. That’s taken off.” and
I was going to fire Comey knowing there was no good time to do it. And in fact, when I decided to just do it, I said to myself — I said, you know, this Russia thing with Trump and Russia is a made-up story. It’s an excuse by the Democrats for having lost an election that they should’ve won.
That’s what Trump actually said.