Something so heinous it would make who-shot-Kennedy a forgotten footnote? Everyone is going to cover that up.
The facts of the matter are probably much closer to what you’re saying, but some people have become so radicalized (including those who got that way due to Trump-era buffoonery) that they can’t accept even the possibility that most of the actors are operating to protect our collective legitimate best interests.
Most of the people in government and the military are not political appointees, are not political hacks, and are doing tough and sometimes high-risk jobs, from a motivation other than evil. On the other hand, many politicians are motivated mainly by political partisanship and will do anything to try to score political points, including trying to cause outrage about what’s being kept secret, even if it’s been explained to them why it must remain secret.
It may very well be that those people genuinely believe that they are acting in "our collective legitimate best interest" and so are not mustachio-twirling evildoers. What they do is still much more harmful to our society than any secret leak would be, for the simple reason that it undermines the fundamentals of a liberal democratic republican society - and if we don't have that, what are we even fighting for?