For example, he could expose the inner workings of the compound or he could provide information to Micheal's enemies - such as the fact that Senator Gearry 'belongs' to him. Not to mention how much he already knows about Micheal, factors that could be used against him, such as the murders he has committed etc.
1) Of course, the inner workings of the compound could not possibly have been know to the two assassins who attempted to murder Michael nor to the person or persons who murdered the two assassins. Oh no.
2) Senator Geary's belonging to anyone was moot long before Fredo's murder.
3) Michael only committed two murders both of which were judicially disposed of long ago. Fredo was not privy to any murders Michael ordered nor would such knowledge, had he possessed it, placed Michael in any legal jeopardy as the attorney's on the Board will so confirm.
