This hardly counts, since they were all bad guys, but In Fallout 2 I killed the entire city of New Redding. I mean every last valid target.
I had been doing side quests for two competing gang bosses. Eventually one of the bosses wouldn't let me refuse a mission. He pulled a gun and shot me.
Thing is, I was wearing Mark II power armor and toting a Gauss rifle. For those who are unfamiliar with the game, these are the best two pieces of equipment you can own: Mark II armor is the next best thing to invulnerability, and the Gauss rifle is the "every time you pull the trigger, an enemy explodes into pink mist" weapon. To either side I had a companion similarly equipped.
The gang boss had a pinstripe suit and a tiny .32 pistol.
I stood for a full minute amid the echoes of the "p-ting" noise his eensy weensy little bullet had made when it skipped off my armor. I couldn't believe he had actually drawn and fired on three dudes who had plasma weapons.
The boss and his two henchmen died in one round of combat. I figured that was the end of it. Nope, the other gangsters in the building had heard me. Naturally they were vaporized in short order.
I stepped out into the street, chuckling at the stupidity of post-nuclear humans....and then I was taking fire from Tommy guns. The entire gang was aware of what had just happened inside. My amusement only increased as I mowed them all down.
Then a prostitute ran up and attacked me with a switchblade. An unskilled, skinny girl with a dull blade the length of my finger thought she was going to dispose of three Iron Man suits. I was laughing out loud as I pulled out the Super Sledge and skipped her down the street like a rock across a pond.
Somehow the other gang got involved in the fight. Maybe they were struck by stray rounds. What I do know is they started shooting at me too.
After twenty minutes of weapons fire, the streets were paved with corpses, and there was nothing left alive to attack me. Quite fun just to know I could do it.
