I’m referring to their DS9 incarnation, not the new show.
I kinda come down on the side of space KGB.
Most of the actions they did, was just standard spycraft that you would see in any war, and the dominion war was as brutal as you could get.
The worst action you could say that Sloan and section 31 attempted, was trying to genocide the founders via that genetic virus plot. Probably okay because the founders were all collectively involved (excepting Odo) in brutal space imperialism, subjecting the entire delta and alpha quadrants to subjugation and enslavement from their home planet, that was the target of the virus.
Neither i think. There’s a lot of theories about it, from the omnipotent illuminati or foreign plot (i think we can rule those out, no matter how much Tal’shiar would like that) through the unoffical hardcore faction in Starfleet Intelligence (this is the most supported one), legitimate black ops division of Starfleet Intelligence, black ops insider cell by some other part of Federation inside SI up to literally one smart guy with many contacts.
I kinda forgot how it was in DS9 though :)
According to Memory Alpha, it was a legitimate section of Starfleet Intelligence in the 22d and 23rd centuries, but by the 24th was a splinter group operating without official sanction.