Rod Dreher’s response  to Trump’s firing of FBI Director Comey seems right to me:
Either Trump is a colossal idiot, or he is hiding something, and is a colossal idiot about trying to cover it up. Either way, I don’t see how anybody can take what Trump has done here at face value.
The first possibility of sheer stupidity can’t be ruled out. When I heard the news last night, I said this:
Removing Comey was probably one of the dumbest things this admin could do, so of course this is what they didchange_me
— Daniel Larison (@DanielLarison) May 9, 2017 
If the hope was that firing Comey would reduce scrutiny on the administration, Trump and Sessions could not have miscalculated more. Many members of Congress, including some Republicans, are going to demand an independent investigation (and some have already called for that), and sooner or later they are going to have one.
Since the administration already squandered what little credibility it had months ago, it is reasonable to assume that Comey was fired in order to impede an investigation of Trump and his advisers  and not for the reasons given by the Deputy Attorney General. Firing an FBI Director who isn’t even halfway through his term would look bad in any case, but doing it under these circumstances is obnoxious and is sure to be perceived as an abuse of power by most Americans. Who thinks that Comey’s successor will be more independent and less compromised than Comey was supposed to be? If we’re being honest, I don’t think anyone really thinks that.
Trump’s larger problem here is that he has proven time after time that he can’t be trusted, so very few people outside his own party will be inclined to give him the benefit of the doubt when he does something as suspicious and controversial as this. Because he has frittered away any credibility he may have had with countless lies and distortions, most people will assume his explanations for his actions are untrue and will assume something else must be going on.