In Fallout 3, when I first started, I was bored and needed equipment, clothes, etc. to sell for precious bottle cap currency. So, I decide to go into the Church of the Atom. Didn't originally intend to, I was curious what was inside, but I ended up saving then going on repeated rampages, attempting to plan a way to use my bat and pistol to murder everyone in the building without one escaping so nobody on the outside would know.
Well, eventually I decided that the old black woman was the hardest to kill, so I crept up behind her in the back room with my bat. One hit to the back of the skull didn't kill her, but my second swat to her leg did. I ran to the door and stood there, holding up my guard, letting the people charge me. When they ran, they ran away form the door. Eventually, I bludgeoned enough people that everyone else, amounting to about four frightened survivors, ran off to huddle in the corners. I picked them off. Having committed religious genocide (out of boredom, no less), I proceeded to take all of their things and sell or use them based on usefulness to me.
Later on, a few levels later, actually, I returned and went upstairs to the bunk area, where the preacher guy was unaware of the nude corpses that had not yet vanished, was to sleep. I sat in the corner and waited. When I came out of the 'wait,' he was just going to bed. I waited three more hours and came out at midnight or something. Regardless of the time, he was fast asleep. Needless to say, he never woke up again. I snuck up to his bed with my well maintained combat knife, using VATS for a sneak attack since he was on his side facing away from me. The strike took off his left arm and tossed his body into the wall (somehow), which then fell a little bit behind the bed. I looted his body, the rest of the building, and left.
So, yes, I committed religious genocide mostly out of boredom, then out of a will to finish what I started. I'm not too proud of that, but, the funny part is that I never would have gotten to my godly character now without the funds from selling items and weapons that killing spree got me. I could have, but the story would have been slower and much more different.
Still, I'm not proud of that, but it's a game so I can joke about it.
