Strzok's participated in or drove the following:
- he was behind investigation into Hillary's illegal email servers and the "accidental" violation of the law that involved deleting, bleachbitting, and smashing of the devices with hammers to erase 33,000 Emails that she was required to deliver to the government (under law).
- after two of Hillary's top aides (Cheryl Mills and Huma Abedin) were both caught lying to the FBI (claiming they were unaware of Clinton’s use of a private email server, and evidence came out that both were 100% aware of them), instead of suggesting criminal prosecution (as is the norm), he gave them and other aides immunity instead. As they did to the guy who destroyed government property (the emails and servers), so he didn't have to testify against who ordered it. 
- After finding obvious crimes like that, he did the only thing a tough G-man could -- he did FBI's fateful interview with Hillary Clinton on July 2, 2016... after first drafting and editing her exoneration letter, not putting her under Oath, making sure there were no recording devices in the room, and then doctored his bosses letter to reduce “grossly negligent” (an obvious crime) to “extremely careless,” (still a criminal offense, but not worded to be obviously so). Being extremely careless with government secrets has ended others careers and put them in prison -- but they weren't the Clintons.
- he texed with his illicit affair partner how much better a President she would be than Trump (and they had to have an "insurance policy" to make sure Trump was never President), and about his collusion with other conspirators (like Andrew McCabe) on their plan. The former shows he shouldn't have been on any of the investigations he participated in -- the latter implied criminal conspiracy.
- NOTE: The DOJ/FBI had "lost" many of the text messages, that they're required to archive by law. Then they magically found them when they were about to be held in contempt of congress and start a bigger investigation into their policies 
- he received the infamous anti-Trump "dossier", and knowing that it was made-up and paid for by the Hillary campaign, he launched a counterintelligence investigation into Russian meddling to allow the Obama administration to get FISA surveillance of a Trump campaign, and then illegal unmask and leak (all high crimes)
- He was the FBI agent who asked Trump's national security adviser, Michael Flynn, about his phone call with the Russian ambassador, Sergey Kislyak, and when he lied -- unlike Hillary's 5 aides, Flynn didn't get immunity, he got the book thrown at him. (That book is titled, "The Double Standards of Hillary Clinton supporters").
- He volunteered to be part of the Mueller Russian Witch-Hunt, that he knew was based on the Fake-Dossier, and he either never spoke up about that source, or Mueller was a polemic that didn't care. Strzok had to be fired once the inspector general of the FBI found evidence of his bias
- Mueller has never been questioned about what he knew about Stroz, McCabe, others, (or the Clinton-Steele "pee-pee" dossier) and when he knew it. But that's key to understanding whether Mueller was "grossly negligent" or part of the illegal conspiracy to have an American Coup and undermine the duly elected President.
- He also had long ties to the Clinton's, that few have bothered to mention 
- And the Wife he was cheating on (Melissa Hodgman), was also a staunch Hillary supporter, though she tried to scrub her Social Media pages of any of that, after this whole mess broke.
- Fired for bias and various other improprieties that came to light.
- Comment on Hillary becoming the first female presidential nominee -- "About damn time!"
- On Russia investigation, "I want to believe the path you threw out for consideration in Andy's office (FBI Deputy Director Andrew McCabe) -- that there's no way (Trump) gets elected -- but I'm afraid we can't take that risk. It's like an insurance policy in the unlikely event you die before you're 40 ..."
- In response to, “[Trump’s] not ever going to become president, right? Right?!”. Strzok replied: “No. No he’s not. We’ll stop it.”
Remember the text about "We’ll stop it." was hidden from Congressional investigators by the FBI, proving the FBI was lying to congress. That was only found out much later, despite having requested it. So not trusting the FBI after they lied to congress and the public isn't just prudent, it's proven to be wise.