James Comey's testimony shows that he deserved to be fired. He has little respect for the authority of the President. He was not willing to honestly report to his superiors on the activities of his department. He has no regrets about the Clinton email investigation, which most Republicans and Democrats agree was bungled. He subscribes to kooky Russian conspiracy theories. He appears to have joined forces with Trump-haters who leak documents and commit other illegal acts to undermine the Trump administration. He was going to spend 4 years causing unnecessary trouble for the President.
Trump thinks like a CEO, and a CEO would not put up with a department head with Comey's attitude.
Comey talked about how the Russians tried to influence the election, but most of that evidence, as documented by the intelligence agencies, consisted of broadcasts by the RT television network critical of the Obama administration. The Democrats and the deep state are apparently very unhappy that they cannot control 100% of the press.
It is not against the law for the President to say that someone is a good guy, or to hope that someone will be cleared. Flynn had already been fired, and Trump had no need to stop an investigation of him for failing to file some paperwork, or whatever he is accused of. Trump was just doing the honorable thing by vouching for Flynn.