A well-intentioned, possibly even heroic, but still dangerous keter.
There's a small (about three meters long) extraterrestrial spacecraft, drone, or weapon (it's not really clear) that showed up in deep orbit shortly after the death of Josef Stalin. It's equipped with a powerful railgun that fires small, non-explosive tungsten rounds onto the earth's surface to eliminate political leaders with fascist policies. At first, the Foundation believes it to be a form of spontaneous combustion, until they start finding shattered remains of the bullets.
The spacecraft actually gives advance warning to its targets, in the form of an email from "null" reading "SIC SEMPER TYRANNIS - [recipient's current latitude + longitude] - [recipient's name] - [a number] - SURRENDER AND YOU WILL LIVE"
Thirty minutes later, the recipient gets the same email, but the last number has gone down by one. If they abandon their current course of action that would infringe on someone's human rights, they get an email reading "YOU HAVE MADE THE RIGHT DECISION" When it hits zero, they're shot where they stand in such a way that there's no collateral damage. (If the target is on an airplane, they're shot as soon as they leave the plane.)
other emails are sent by "null" to the victims and/or resistance movements of his targets, such as "TYRANT ELIMINATED [date/time/coordinates of assassination" - PAYMENT DUE FOR SERVICES RENDERED: ABANDON FEAR. KNOW THAT YOU ARE PROTECTED. KNOW THAT YOU ARE LOVED."
In addition, the scp has a set criteria for the various "tyrants" it seeks out - it only intervenes as a last resort, and will generally ignore dictators who are set to be deposed and/or executed, such as Ceaucescu, Karadzic, Gaddaffi, and Saddam Hussein (though the latter has received several "SIC SEMPER TYRANNIS" notices over the years, causing him to scale back his plans as needed.)
Big end twist: the entire O5 council gets a "SIC SEMPER TYRANNIS"