Anyone can have bad karma, but it takes a certain person to be seriously evil.
Enter Megaton, and promise Lucas Simms that you will disarm the bomb.
Go to Moriarty's and promise Burke to detonate the bomb.
Get to know the people of Megaton, do their quests and help them.
Murder Lucas and steal his hat and armor, and run around town screaming "Hey guise look I'm Lucas Simms!"
Optional: get Cannibal perk and feed on his flesh.
Play soccer with his head in front of his son.
Break into Lucas's house at night and rob his son of all his possessions and sell them to the store, making sure you get all the food.
His son now has no food and will surely starve to death in time.
Wait 2 weeks in game.
Go to Tenpenny tower, put on the Tunnel Snakes jacket (if you helped Butch) and go into third person mode and feel like a total badass.
Press the button.
Laugh as the bomb detonates.
Slosh wine in a glass and joke with Tenpenny.
Sorry for the long post