I used to work with a former FBI agent who left to become an attorney. The heart of the FBI is ultra conservative, and this goes back decades. COINTELPRO was the nadir of FBI political activism. The shock over the Church commission and realization that the FBI was actively working to infiltrate subvert left wing activism pushed down political meddling by the agency for a while. But the ultraconservative heart of the agency has not been able to resist the urge to become political. After 9/11, the FBI used terrorism as a pretext to take action against anti-globalization and environmental groups. The FBI also tracked anti-war protesters and the Occupy movement.
The FBI hates serving under Democratic administrations. They see the Democratic leadership in the US AG's office as being soft on crime and making their lives miserable. The FBI comes after Democrats hard on everything and everything. But they then tread lightly with Republicans. Jack Abramoff should have taken down about half of the Republicans in congress. Instead, only a bunch of staffers and one House rep got indicted.
The Clinton email scandal has been a political witch hunt from the beginning. The FBI finally had to admit it after coming up with nothing in their investigation. But Comey and the FBI found a pretext to jump back in when they found more emails in the Weiner investigation. The mere existence of more emails did not in and of itself merit re-opening the investigation. They had already been through hundreds of thousands of emails and found nothing. But Comey new that he could throw a wrench in the Clinton campaign while claiming "transparency" by re-opening an investigation that had already been a complete failure.