There was an investigation going on an firing the guy heading that up just as it started to take off was bad for Trump's image
He didn't fire the guy leading the investigation, that's just a narrative being pushed to ignorant people.
The FBI director is a political appointee and like all political appointee's he doesn't lead ***. Under neither him are many division heads, and under them are various task forces and inside those task forces are the people who lead investigations.
Firing Comey has absolutely zero impact on the Trump investigation, zero, zilch, absolutely nothing changes.
Following said firing up with several changing explanations as to WHY Comey was fired was also bad for Trump's image.
That's more of that political theater from the left directed to ignorant people.
There is exactly one reason Comey was "fired", he didn't do what's traditionally done and submit his resignation. Whenever a new administration takes office it's normal for the previous administrations political appointees to submit resignations to make way for new political appointee's. Congress can not approve a new political appointee until there is a vacancy, so to prevent an embarrassing the old resident resigns and the deputy takes over as a temporary acting head. If an agency head refuse's to submit a resignation then the President must forcibly remove them, aka firing before a new nomination be submitted to congress.
Congrats on getting duped into thinking a normal procedure was nefarious. Seriously look at the time lines of FBI directors, they change within a year or so of a new administration as the President wants someone who supports their vision instead of the person who supported the old administrations vision.
FBI director is the same as CIA director, the Presidents cabinet and the various political appointees under them, all people subject to congressional approval and changed out every time there is a new administration. Director Muller was kind of an oddball lasting from Bush into right past the start of Obama's second term. Prior to him Director Pickard was the last FBI Directory who was an actual agent, the ones afterward were just justice department lawyers.