Admittedly, it's hard to argue that Comey hasn't harmed public faith in the FBI, because he has. His botched handling of the investigation into former Secretary of State Hillary Clinton's use of a private email system was condemned by politicians in both parties as well as many former Justice Department officials. It broke with longstanding department precedent and practice, and history will record that it helped to alter the election's outcome. In the end, Comey put a higher priority on his own reputation than on justice, and in the end both his reputation and justice were harmed.
Trump, however, was angry with Comey for other reasons. He had made it clear during the campaign that he wanted Clinton prosecuted, and that he strongly disagreed with Comey's conclusion that prosecution was not merited. "Lock her up" became a popular chant at Trump rallies, and Trump promised repeatedly that if elected he would overturn Comey's decision, which is power that even the president does not have.
As recently as six days ago, Trump was making it quite clear that he still had not gotten over it.
Nobody expects that a new FBI director will reopen the Clinton case. That would be grounds for potential impeachment. But clearly, both as presidential candidate and as president, Trump has not shied from attempting to put immense and highly inappropriate political pressure on the FBI and Justice Department to produce the outcome that he demands. There is no sign and indeed no hope that he is capable of changing his ways at this point. That's who he is.
In his letter announcing Comey's firing, Trump spoke of the need to "find new leadership for the FBI that restores public trust and confidence." Public trust and confidence in the investigation into Russian meddling into our election -- meddling that Trump continues to publicly deny -- is equally important.
Only an investigation that is clearly free of political pressure from Trump and his appointees will be able to produce such an outcome. Continued faith in the system absolutely requires it.
¹Here is what Comey said in sworn testimony to Congress as recently last month:
"I have been authorized by the Department of Justice to confirm that the FBI, as part of our counterintelligence mission, is investigating the Russian government’s efforts to interfere in the 2016 presidential election and that includes investigating the nature of any links between individuals associated with the Trump campaign and the Russian government and whether there was any coordination between the campaign and Russia’s efforts. As with any counterintelligence investigation, this will also include an assessment of whether any crimes were committed."