I've had some pretty crazy escapades of mass-murder in my time, with no clear ability to judge one as more violent and bloody than the next, so I'll just post those that I recall.
First, and quite a while ago, I was having some adventures and found myself in a tigerman town or whatever. I was testing whether it was really, truly impossible to depopulate a town, so I immediately set to work wiping out the population while they slept. All in all some 600 tigermen died by my hands, yet my site and pops file export showed only 150-200 or something like that as the total population, so I concluded correctly that it is completely impossible to depopulate a town since, I think, 31.17.
Even before that, in 40d; I chose to go all-out evil, and wiped out a civilization or two, because why not. I don't know how many I actually killed, but I know they STAYED DEAD, and therefore it is far more horrifying than anything I could do in recent versions. I and my companions picked a fight with some foreign civilization's demon leader and murdered the entire population of a town in one day and one night, because everybody kept trying to get to the inn, which we were trapped inside of, because everybody wanted to get into the inn!
Recently, however, I came upon a cave full o' kobolds, and being an idiot, got spotted right as I stepped into the vast horde at the bottom of the cave, where it meets the caverns. Being man of various magical disciplines, the fight was very one-sided even though I had shown up alone with little armor or notable combat skill. I cast fireball and lightning like nothing else, and used my shape-shifting to keep myself alive whenever I got injured. In the end, there were only ‼kobolds‼ and ‼bones‼. Of course the kobolds were rightfully pissed about it and chased me down repeatedly until I finally escaped their territory. Then the next cave over decided they hated me as well.
The best yet, was when I tried to get a legendary adventurer to commit suicide via marketplace brawl. I had gone on a rampage through a city and very, very brutally slaughtered dozens of people whilst sneaking from house to house. Unspeakable things were done to them. Then I came upon a market and decided to end it all here. Which didn't work. That adventurer had become so legendary in every profession that she just swam through the sea of peasant blood and produced more of it. They just kept coming from all sides for 20 minutes or so BUT THEY JUST KEPT DYING. In the end I'd slaughtered like 3 market districts full of merchants and dogs, and I just gave in to starvation...