As I write, we don't really have any idea why Donald Trump decided to fire FBI Director James Comey, but there is widespread agreement that it's inappropriate for the president to have done so, even more inappropriate for him to have done so the way he did it, and considerable speculation that Trump means to have the investigation into the Russia scandal suppressed -- speculation egged on by the practically insane letter Trump released dismissing Comey, which directly referenced that investigation.
Regardless: If it was necessary before to have a special prosecutor named to look into the Trump-Russia scandal -- and it was -- it's absolutely imperative now, along with a (separate) Senate select committee.