"Those were lies, plain and simple"

Former FBI director James Comey testified in front of the Senate Intelligence Committee yesterday. He made a statement available to the public prior to his witness testimony which was analysed in great detail by Lawfare: 

Throughout the document, Comey reports extreme discomfort with Trump’s behavior generally, and this aspect of it particularly. At that dinner, Comey felt compelled to tell the President that he was not “reliable” in the way politicians expect. He reports that the President’s efforts to engage him in a “patronage” relationship “concerned me greatly, given the FBI’s traditionally independent status in the executive branch.” He describes the interaction as a “very awkward conversation.”

Remarkably, even before that meeting, Comey was so uncomfortable with Trump that he had already begun writing memos recording every interaction he had with the President and sharing them with the FBI’s senior leadership.  

Lawfare: Initial Comments on James Comey’s Written Testimony, June 7, 2017

In the hearing itself, Comey gave more details regarding what he thought were the reasons for his firing (not the initial justification by Trump - his handling of the Clinton emails - or that he had lost the trust of the FBI, which prompted the line quoted in the title) and his decision to write memos (and make them available to the public) about his meetings with Trump. A sidenote to his testimony regards that he was approached during Clinton email server investigation by then-Attorney General Lorette Lynch and asked to refer to the case as an investigation, but as a "matter".

Beyond the question of whether President Trump has broken any laws, Comey's testimony reveals that he considered his actions to break with the norms of how the President of the United States approaches the FBI and investigations: 

But the question of what went wrong in that White House encounter is at heart not a legal one. What’s most insidious here is the president’s violation not of any law, but of norms—what we collectively define as acceptable behavior by those entrusted to lead.
No law stops the president from discussing a criminal investigation with the head of the FBI. No law stops him from ordering the end of an investigation, or from firing an FBI chief. As Comey himself said in his testimony, “As a legal matter, the president is the head of the executive branch…I think he has the legal authority.” But, as Comey also said, “we have important norms against this.” 

Quartz: The big takeaway from Comey’s testimony is the awesome and arbitrary power of the president, June 8, 2017

In its report on the hearing the New Yorker sets the stage with Comey's outrage over what he considered to be defaming: 

From his opening words, Comey made clear that, as a private citizen, he is no longer confined to the narrow, self-edited comments that defined his years at the Justice Department and the F.B.I. He delivered an opening statement that offered a full-throated defense of the thirty-five thousand men and women at the F.B.I., in effect calling on them, from beyond the professional grave, to carry on their work without fear of White House intimidation. Comey also bluntly framed the Trump White House as a duplicitous agent of political obfuscation. Of his firing, Comey said, “The Administration chose to defame me and, more importantly, the F.B.I. by saying the organization was in disarray, that it was being poorly led, that the workforce had lost faith in its leader. Those were lies, plain and simple.” 

The New Yorker: Comey's Revenge: Measuring Obstruction, June 8, 2017

And then goes on to outline the possible legal consequences of the testimony - which include both the option of obstruction of justice, when Trump told Comey "I hope you can see your way clear to letting this go, to letting Flynn go", to trading an official act for personal gain, for Comey's description of Trump's actions as trying to draw him into a "patronage" relationship. The Atlantic outlines the lines of defense that were open to Trump supporters against Comey but failed. 

In one reaction to the testimony, Benjamin Wittes writes for Lawfare that the main issue here is what it says about Trump as the President of the United States: "we have a president about whom the FBI director, after meeting him once, began writing memos to file, because he did not trust him to tell the truth about their interactions or to respect the functioning of law enforcement".