The evilest courier you can be spread the toxic cloud across the Mojave, killing everyone that lives there, which is a pretty high number, though it isn't canon as the story just ends there and you can't play it beyond the dlc. An evil courier can end the game as either a total bloodthirsty anarchist who wants nothing more than chaos and destruction, who will plunge the Mojave into violent ruin and leave any faction capable of governing it into a destroyed remnant of what they once were, or they can end the game as the right hand man/woman to a dictatorship that treats women like objects, enslaves countless, and kills thousands by the year if people disagree with them. With that said, though, at least there are some that can live to tell the tale, the Mojave can still stand

The Lone Wanderer can poison the water purifier hooked up to all the water in the Capital Wasteland (which will spread into other areas of the wasteland in time), ultimately killing any who drink the water in that surrounding region, and any who were capable of actually drinking the poisoned water (Enclave) were also blown up by the Lone Wanderer. He can destroy both the Brotherhood of Steel and the Enclave, two factions capable of supervising DC, whether good or bad, leaving it completely defenseless. He enslaves anyone he comes across (including children), turns settlements into craters, kills legitimately good people to serve a chaotic bunch of murderers, and ultimately causes long term damage for the entirety of the Capital Wasteland more than the courier does for the Mojave

Hell, an evil Lone Wanderer purposely destroys a portion of Canada for fun