Just pretend the game doesn't end. It's better that way.
This.
I reloaded my save from just as I entered [spoiler]Project Purity[/spoiler] and ran back out and continued on playing the sidequests. As a sort of retribution for the bad ending, I fast-traveled to Tenpenny Tower and killed every one of those smug pricks inside.
It was here that I realized that the karma system is a little dodgy. When I killed the residents and guards (the same ones that hired me to kill the Ghouls), it dinged my karma, but when I killed Tenpenny and his assistant, I apparently was morally justified. I dunno. I think maybe the entire idea of putting a moral or ethical judgment on the actions of a character dropped wholly unprepared into a lawless and hostile postapocalyptic world is a bit unsavory.
Or maybe I just don't like games judging
me. Who knows?