Tbh, the sewers aren't in all that of a horrible place right now. Yeah, there are dangers there with NPCs. But for the average Civilian/Rebel, the danger isn't that high so long as an admin isn't down there spawning NPCs to get you. My first day on the server consisted of me and a few other civvies walking around for about an hour down there, slowly getting that spore to build up, but only ever encounter one NPC which we just ran from since we couldn't fight it. It was dangerous, and we got afraid when we couldn't escape, but we had that sense of danger. An uncurated experience.
On the other hand, you got CP Sweeps of the sewers which, from my experience, are mostly reactionary. I; E: Biotics spotted, escorting CWU to fix tech, or just culling necrotic/parasite populations. These are honestly more so 'hosted' at times by the admins of the server and are meant to incite a form of danger to the CPs', though other times, like today, you do get that unpredictability with the players being down there at the wrong time. And tbh, it's fine if it's like an admin event char rping a fight and pking someone, or a TBC fight with that event char/player char - but when it comes to NPCs, it can feel cheap to the player - especially if Garrys Mod Jank gets in on the deal and kills off your character just because a gameplay mechanic didn't work properly - then being told "its either NLR or PK"., it can be fairly off-putting to a person.
GMOD is known for its jank, especially with scripts and NPCs, so dying to an NPC in the sewers in a fashion that makes no sense may be off-putting for people and can seem really harsh when you just got unlucky thanks to a buggy npc that all ur stats and such are now gone and so is the chars story, less you get an appeal. IMO, unless it's curated, a single NPC should rarely, if ever, be able to PK a person. As the ramifications of that are just huge. The player may feel their time was wasted, they get annoyed, all that - and I'm sure if appealed it'd be fine. But it's still an off-putting thing to have to do, and know that it can happen at any time thanks to poor luck gameplay-wise.
TLDR: The sewer as a place of fear is a good idea, and there should be danger - some NPC's sprinkled here or there to spice it up, players fighting other players, all that - but NPCs themselves shouldn't really be able to PK a character based on gameplay mechanics, as it's a one-time situation not curated that can end a person's char' that they've worked on for weeks, if not months, and spoil the enjoyment. And can be prone to messing up, without an admin there to witness the issue.