Trump was angry that Comey would not support his baseless claim that President Barack Obama had his campaign offices wiretapped. Trump was frustrated when Comey revealed in Senate testimony the breadth of the counterintelligence investigation into Russia’s effort to sway the 2016 U.S. presidential election. And he fumed that Comey was giving too much attention to the Russia probe and not enough to investigating leaks to journalists.
The known actions that led to Comey’s dismissal raise as many questions as answers. Why was Sessions involved in discussions about the fate of the man leading the FBI’s Russia investigation, after having recused himself from the probe because he had falsely denied under oath his own past communications with the Russian ambassador?
Why had Trump discussed the Russia probe with the FBI director three times, as he claimed in his letter dismissing Comey, which could have been a violation of Justice Department policies that ongoing investigations generally are not to be discussed with White House officials?
https://www.washingtonpost.com/politics/how-trumps-anger-and-impatience-prompted-him-to-fire-the-fbi-director/2017/05/10/d9642334-359c-11e7-b373-418f6849a004_story.html