Posted this on the forum, but I'll put it up here as well for reference.
I did a quick run through the game to see who Yurt killed if I freed him as early as possible. I think this was in NG++++.
Ostrava had already died in 1-1, so he was never part of it.
- Freed Freke, killed Fools Idol, freed Yurt. Killed Armor Spider, fetched Patches.
- Nexus: Yurt arrived, two dead bodies appeared.
- Killed Adjudicator, freed Urbain, and Patches again.
- Nexus: Yurt killed Freke's Apprentice.
- Killed Tower Knight, freed Biorr and Yuria.
- Nexus: Yurt killed Acolyte of God (male worshipper).
- Killed Leechmonger.
- Nexus: Yurt killed Patches, a flame at patches location containing the thiefs ring. Talked to Yurt after Patches was dead, and he said he only needed to kill me (ran away before he completed the sentence so he didn't aggro).
- Killed Flamelurker.
- Nexus: Yurt killed Biorr, brushwood set available at his location.
- Killed Old Hero.
- Nexus: Yurt killed Urbain, saint gear at his location.
- Killed Dirty Colossus
- Nexus: Yurt killed Freke, sage gear at his location.
- Killed Maneater
- Nexus: Yurt killed Yuria, her drops available at her location.
- Killed the rest of the bosses, no more npc's were killed.
All unlocked npc's are gone. Blue Phantom, Worshipper of God (female), Blacksmith Boldwin and Stockpile Thomas remains. The same npc's that are present when you start a new game.
I finally killed Yurt and spawned Mephistopheles, and she still treated me like I had done the killings, so I killed her as well, and got her usual drops.
I've also had some inconsistencies when killing two or three bosses in a row, without leaving the world, and upon returning to the nexus only one was missing. So it may be dependent on boss kill + return before he kills another one.