It depends on several factors: how the playthrough is going, whether i've just saved before hand and it's a designated spree time before I reload, and whether the NPCs have earned it.
Last time I played Fallout 3, I thought that before I blew up Megaton, I'd kill everyone there that I could first... so I did, then blew it up. I also killed everyone on that bridge cos Lucy West's kid brother attacked me.
If the NPC isnt valuable in any way to me, as in isnt a shopkeeper or something, or sells something rubbish, or I can find a better alternative, then I may well kill them for a laugh. On Oblivion, I don't kill randomers all that often. Generally, in any game, I may kill an NPC with an innovative method. I never got too far into Fallout 3 because it was on my housemate's 360, but I wiped out all the people I could get away with either creatively or in a massively outnumbered shootout. I remember killing the ghoul who wanted to get into Tenpenny Towers by leaving a small stash of mines by his and his wife's beds, then shooting her with the BB gun so she got up and blew them both to shit.
For Oblivion, I made a frenzy/rally spell that, when cast on a homeless dude, and some other NPCs such as the adoring fan, it causes them to run off and attack people, which is pretty funny when you hit someone strong with it because then you get a lot of bodies to loot. The best place I've found to do this in is the Arcane University, so long as you can escape, because everyone killed is generic respawnable scholar or apprentice, and they often drop reasonable enchanted items which may be either useful or sellable
