There are many examples of covert malevolent acts in the halls of power throughout history, there was a list of confessed false flag operations on here just the other day. Look into any book about the history of military intelligence and you will find all sorts of nasty adventures. Then there's COINTELPRO, MKULTRA, the FBIs attempt to undermine the civil rights movement, it goes on and on. What would be truly irrational would be to believe that this stuff doesn't happen in the present.
So now I try to evaluate each "conspiracy theory" on its own terms, lest I blind myself to the one that turns out to be real.
(Note: I still don't actually believe in any specific tinfoilery. I just try to evaluate it honestly. Almost all of it, I still dismiss immediately. Most of the really juicy stuff is right there in the history books anyway.)