1) Why can't we compare NPCs to AMPCs and how the use of one affects the perception of the other? On the contrary, let's.
Vampire NPCs exist in dungeons or in out-of-the-way Mist locations where they are sufficiently isolated that their appearances can be controlled and bound.
Wererat NPCs do admittedly spawn everywhere, but this is a pretty accepted feature of wererats. The use of Wererat NPCs is thematic.
Red Widows on the other hand are meant to be pretty mysterious and secretive, AFAIK. So we can consider whether the use of Red Widow NPCs works with or against their themes.
2) Handwaving something about the Dark Powers is missing the point here. The Dark Powers are merely the IC reification of OOC DM fiat. Just because the Dark Powers/DMs can do anything does't mean that they should. The real question is: does having a bunch of Red Widow NPCs appear in the middle of the Marchand and decimating the local gangs support the themes of the Red Widows or Dementlieu? In my opinion, without dressing it up in Dementlieu's themes, no, it shouldn't be happening; there's no deception here and IMO it risks spoiling the Red Widow mystery.
3) Sure, mostly vampires, along with the occasional ghoul, mummy, and death knight (Hi Doodles!). By and large, none of these get used in Dementlieu as NPC mobs, so there's no risk of them undermining any of those themes.
4) I think we all acknowledge that their not despawning is a bug. That's not the issue.
5) There are multiple strands to this argument, some of which have already been broached.
- I think cranking up the NPCs in the Marchand, especially with capabilities like See Invisiblity and better stealth killer capabilities, at least merits discussion. Not all areas of Dementlieu are high level, and Charlotte's Daughters are top level spawns. There's a little nuance at play here. I know plenty of low- to mid- level PCs who used to level themselves fighting in the Marchand at night. Heck, the Dev who created the Charlotte's Daughters used to do this!
- Even if it's decided that the spawn level is not the issue, there's still the issue of theme; monsters in Dementlieu should be more hidden (see point (2) above); like I said, if we're going to keep the Red Widow spawns in the Marchand, better to dress them up as another gang, rather than show them up as monsters right in the middle of the Marchand.
- There is a world of difference in Dementlieu between PC Heros/Anti-Heros/Villains going out and killing thugs, and NPC monsters doing the same. When PCs (or DMs) upset the setting, that's story. When random spawns do it, it's potentially depriving a PC or a DM of the space to do the same.