Trump said. “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 have won.’”
Democrats see that quote as a smoking gun. Trump admitted to firing Comey to stifle the Russia investigation.

or......alternate theory.......

If Trump did not collude with Russia, then Trump would know for sure, that an investigation would yield nothing. He would also know, for sure that no evidence of collusion exists. Therefore, Trump could conclude that Comey is conducting a politically motivated investigation with no evidence. Fire-able offense.

Then...two days later, a highly respected bi-partisan deputy attorney general made an extensive written recommendation to fire Comey based on an overwhelming lack of public confidence related to his bungling of the Hillary investigation.

A freshman communications student can tell you that the latter plays better as an "official message".

And then later, a jubilant, bombastic, and egotistical Trump couldn't help but drop a brag about HE made the decision himself based on his own evaluation of Comey's incompetence.

I'm not seeing a crime there.

The left's alternate theory is that Trump got lucky that Rosenstein wrote that memo because Trump had already made up his mind to obstruct justice. Then, in an interview with CNN Trump had a brain fart and accidentally confessed. C'mon. He's not dumb!!!