Maybe you’re a kid killer - someone who took pleasure in Fallout 1 and 2 clearing out the little pickpockets with whatever weapons you could. That said, it’s not able to harm the spirits of Moira who somehow survives the blast, bless her. The game recognises this as well, with the negative karma you reap being colossal. It’s a terrible act not only because you have many steps to get through in order to push the button, but you have to watch your choice unfold.
The first thing that many people will think of as a true evil act in the series is detonating Megaton's bomb in Fallout 3, as it takes a special sort of a-hole to go through with it, especially given the reward: an apartment in a ghoul haters' hotel, one with a henchman who makes the Nazi from Indiana Jones look like a social butterfly.
