Star Trek Deep Space 9 - Section 31
Why Section 31, out of all the antagonist groups from Star Trek? They're merely humans with money, that's all. Why not pick something more appropriate, even outside of Star Trek, like others who chose the Reapers?
Simply because Section 31 is terrifying in the way it's almost possible. Mere humans who are fantastics with no sense of morals and the money to accomplish anything. They have no qualms with killing anyone, interrogation, etc, as long as it ensures the Federation's existence.
But another thing is their interrogation is through Holodecks. An entire DS9 episode revolved around how they are able to trick a person into believing the holodeck was real, that they never entered it in the first place and allowed them to create a story to their liking to force the person to give up information. All they know is they fell asleep, woke up, and immediately everyone was slightly different and the world around them was crumbling.
But finally, even when you break free and figure out it's all a holodeck and fake--do you really? How can you know everything else is real? What if when you say "Computer, end program" that it's actually part of the program of the computer to pretend to end and instead make a new version?
The possibilities of how deep into your mind these fantastic can go, and will go, is terrifying to consider. They don't just kill you, they don't tear you apart mentally. They let you do it to yourself.