When I was doing that Dark Brotherhood quest where you have to kill few people in locked house, that one guy just suddenly attacked to another guy and they killed each other. And when I was in bar, Elsa God-Hater suddenly attacked to that one Fighter Guild guy there and then everyone who was in the bar killed her. Odd...

As for the DB quest, it's either a glitch, or scripted to happen, I've heard of it happening with others, but it's never happened to me, so I can't explain.

As for Elsa God-Hater, believe she's a plain clothes Mythic Dawn Agent, and attacks you on sight after reaching a certain point of the main questline. As for the Fighters Guild guy, since he likes your character, he'll help you out in the fight.

In the Whodunit? Quest, if I recall correctly, you can lower the disposition of the party guests towards the other party guests, to the point where they hate each other and try to kill each other... If that makes sense.

The same thing happened to me with Elsa God-Hater, though I still haven't done any of the main questline. Also, I went back there a while later, and she was alive again.

NPC's that have a low responsibility, that eat, and do not have a respawning food supply in inventory will go out stealing food when their script triggers they should eat. A notable example off the top of my head is City-Swimmer in Bravil.
If they get spotted during a theft attempt, the victim will attack them. Other people may join the fight. This can cause sudden NPC death.

Ive heard many tales about Else God-hater being involved in brawls. Its just one of those things that happen in the wonderful world of radiant AI Like that time Glarthir attacked his 'spies' even before I started the quest.

I tend to bribe people so they have money to buy food, but I dont even know if it works lol.

Shameer is like this too. Bribing doesn't work on him, I have to actually leave food laying around on the ground. I don't know if that works either. :shrug:

As is Big-Head, but he steals forks.

As I walked around in the cites, I have seen people turn into (what I think was) Mythic Dawn agents. They have armor and everyone around them attacks. They are eventually killed but sometimes others get killed as well. I saw Else God-hater getted killed this way. When they are killed, they revert back to their old cloths – you can always plunder the bodies – seems a little morbid but free stuff is free stuff.