I see a lot fo people supporting this but what about others that are happy with the status quo? The idea of killing NPC stemmed from the fact they previously did actions to annoy the players like opening doors to let zombies in or block interactive items like chest by standing in front of those, arguably the Goblin Tinker is the most grievous example by taking two platinum to reforge an accessory to menacing.
It seems fitting to tie in boss summoning with NPC killing because there's a darker feeling to it than replacing it with normal summoning items, some of us appreciate that. If this were added there'd be no toggle option since it'd be so pointlessly niche.
Why does everyone dislike the Guide
This also doesn't support NPC equality, which is odd. The reason players hate the Guide more than the Clothier is because one NPC isn't enough to reduce spawn rates for monsters so he ends up getting in the way, but the exact same thing would happen if you started with the Clothier or any NPC for that matter. It's not the "Guide" who's frustrating it's the "NPC programing" that is.
For those of you who state "sure, why not" just know there are some of us that would mind. I get people may want alternatives to murdering their NPCs, which should include not killing the Guide too, but it doesn't really add anything to the game to change it, so this is purely a matter of very minor options for both sides of the argument.
I think protecting thematic elements in the game is a net positive, it keeps the game's character unique.