@katsung47,
I think you are largely correct, although it is a mixed bag. I suppose there is a lot of good that the FBI does, but yes, they are a very powerful institution that often breaks its own laws and runs afoul of its publicly stated duties. Of course there are always 'good reasons' why - national security and the like. I think the potential for damage is directly proportional to the amount of oversight/accountability enforced and that oversight is proportional to whoever is in charge at both the executive level of president and FBI.
Like the CIA and Military, these organizations have a bizarre relationship to the Executive. They are ostensibly under its control, but in practice behave as independent institutions*. Contrary to the "commander in chief" narrative, I believe there is only a dotted line of management to the White House. The same goes with other law enforcement entities as our police and sheriffs. Which explains why it is so difficult to enact Federal change to these institutions..
*Of course executives could take a preternatural interest. Trump could take and since they he is, on paper, in control, make significant change if he pushed hard enough. If they are, however, "on the same page" that could spell disaster for many. The effect of the executive is directly proportional to their interest, personality and similarity of goals..