And, I admit this statement may be controversial, but you need police for a functioning society. People need someone to call if they get threatened or to deal with the aftermath of a murder. Without police, people would demand something that looks very similar to a cop.
You don't need the FBI. It's creation was linked to prohibition which we didn't need either. It's modern form came to be under Edgar Hoover who used it to facilitate Japanese internment, to "purge alleged homosexuals from any position in the federal government" (quote from Wikipedia) and domestic surveillance against civil rights leaders.
They were the agency that should've been responsible for stopping 9/11. Stealing again from Wikipedia, "government documents showed that both the CIA and the FBI had missed 23 potential chances to disrupt the terrorist attacks".
They do help convict murderers though. "Nearly every examiner in an elite FBI forensic unit gave flawed testimony in almost all trials in which they offered evidence against criminal defendants over more than a two-decade period before 2000... the cases include those of 32 defendants sentenced to death". [0]
Either way it shouldn't need to be the only evil left in the world before it's dealt with.
[0]: https://www.washingtonpost.com/local/crime/fbi-overstated-fo...