It depends on how bad it is. For example, if it turned out that a Secret Service officer fired a stray round that hit the President then I don't think we're going to see that made public ever.
I'm quite sure that's why Trump is ordering this release.
He's explicitly at war with the deep state and existing institutions. Trying to tear them down and seize power. If these files make the FBI and/or other TLA(s) look bad, further public eroding trust and support, he wins.
The Republicans are also starting to win back non-white voters. Shedding light on the MLK assassination certainly does not hurt that effort.
If the FBI had really done it, I doubt they would have left classified records saying they had. Most likely, either the records would have been destroyed long ago, or they never would have created any in the first place
Other possibility: they really didn’t do it, but there’s some classified record which sure makes it sound like they did. e.g. some record of J. Edgar Hoover joking about doing it
I think there’s a big difference between “random FBI field agent decides to murder someone” and “FBI director asks his deputy to murder someone”. Attributing the murder to the FBI as an institution makes a lot more sense in the second case than in the first
Legally, a government agency like the FBI is immune to criminal prosecution. Its officials can commit crimes as individuals, but it can’t commit a crime as a government agency because government agencies are excluded by legal definition.
But, it still can be judged guilty or innocent in the court of public opinion and the accounts of future historians
I don't think the majority of the public is aware of the extremely devious stuff the FBI has done w.r.t. civil rights leaders etc.
Even if nothing substantively new is released it gives him a chance to rant about the FBI as part of his mission to seize power and discredit the "deep state" while cosplaying as a supporter of truth, justice, people of color, etc.