Peter Strzok was fired by Robert Mueller. He was a key FBI agent in the Russia investigation. He also happened to be involved in the exoneration of Hillary, targeting Flynn and he was also caught sending anti-Trump text messages.
Strzok wasn't your average FBI agent. Look at all he was involved with.
Per Daily Wire:
He Interviewed Hillary Clinton And Helped Exonerate Her. Not only did Strzok lead the investigation into Hillary’s email server, he interviewed Hillary herself. He also drafted the language responsible for allowing Hillary to escape prosecution — instead of labeling her behavior grossly negligent, he classified the behavior as “extreme carelessness.”
He Was Involved In The Investigation Into The So-Called Russian Dossier. According to Fox News, Strzok briefed the House Intelligence Committee on December 5, 2016, but an informant told the committee that Strzok was allegedly obstructing a probe into the dossier; Strzok was reportedly involved in the transmission of the dossier to the FBI.
He Interviewed Mike Flynn. On January 24, Strzok and one other agent interviewed Flynn, according to an intelligence official. According to one source, Strzok’s boss, Andrew McCabe, contacted Flynn by phone and said that “some agents were heading over (to the White House) but Flynn thought it was part of the routine work the FBI had been doing and said they would be cleared at the gate. … It wasn’t until after they were already in (Flynn’s) office that he realized he was being formerly interviewed. He didn’t have an attorney with him.”
It's not clear exactly why Mueller fired Strzok. He was a top agent that appeared to be tasked by Comey to help get Hillary off the hook. The fact that Strzok was involved with every area near the collusion and letting Hillary off the hook, along with his known biase, shows that this investigation has been thrown off the rails.