Currently I'm at the beginning of the Dark Brotherhood questline (just re-visited my old cell at the Imperial Prison and exchanged a few words with an old friend), and back at the sanctuary, I openly watched Ocheeva killing Maria Antoinette, and right afterwards, when spoken to Ocheeva or anyone else, there is no topic about it. I've been avoiding any spoilers as much as possible, but I do vaguely know that the quest line eventually will involve asassins amongst assassins and most likely I will have to investigate and find the killer - and the mystery is now solved and spoiled before it even has begun. My question is - did I experience a glitch, did I see what my character wasn't supposed to see? Should I reload a previous save and avoid my character being actually there (not that Ocheeva was bothered by my presence anyway) when it happens or just carry on? Or it really was supposed to happen in my presence?

Non of the brotherhood is supposed to kill each other until much later in the quest line, youve run into a weird glitch.

Its possible she took something that belongs to druja.

Eww what should I do, how do I prevent this from happening? Should I put some certain items in Ocheeva's containers? Antoinette didn't have anything that doesn't belong to her on her corpse though.

I'd check your mods. It sounds like you've got something that's interfering with their AI. There's no reason for members to attack one another. There is plenty of food around the sanctuary, which is the usual reason for NPC's to attack one another. Check the sanctuary - if there isn't food lying around, then see if you have a mod that removed it. If you took it all, then make sure you aren't running Harvest Containers.

If you're running Harvest Containers and you don't leave food around for the NPC's, then they might try to steal it from each other. There is a bug in the game engine that prevents NPC's from accessing containers that have a script on them, which all containers will when you're running HC. We have confirmed the bug and Scruggsy was looking at a fix before he left the modding scene. If a new version of OBSE is released, then there might be a fix for this issue in it. In the meantime, make sure you leave some food for NPC's if you're running Harvest Containers.

This also fixes CitySwimmer in Bravil. Leave some food laying around for her and she won't get killed. She'll try to steal it otherwise and gets killed by the guards. Not an issue if you aren't running Harvest Containers though.

Thanks. I loaded Ocheeva's barrel and table with various food and wine, hope it will keep her happy.