If you're like me and couldn't get the Kill'em All mod to work with your recently updated game, this patched version was made for Steam v1.31. Quest NPCs are protected as well and the now non-essential merchants drop their inventories on death. This works with DLC as well.

Credits go to the original author (skacikpl) - Kill'em All - Ability to kill everyone  
And Credits go to KoteBegemote for GOG Version of Mod - Kill'em All - GOG Version

Original description:
Now you will be able to kill (or at least knock unconscious) any common NPC you encounter.
If you load an existing save after installing this mod, you will need to meditate for a few hours or fast travel.

Requirements:
Updated to work with Patch 1.31 Steam Version of the game! I haven't tested it with the GOG Version, but  it might work. KoteBegemote made a nice GOG version if you're interested.

Installation:
Download the mod and unzip it, put modKillemAll folder into your Witcher 3 installation folder\Mods.

PRIMARY VERSIONS:

Kill Em All + Quest NPCs Protected
Kill Em All + Quest NPCs Protected + Mortal Guards

UNSTABLE BUT FUN VERSIONS:

Kill Em All + Mortal Guards - But Enemy Aggro is reduced
Kill Em All + Mortal Guards - Everyone is Aggro (We call this the Let's Fighting Love Edition of the Game)

FAQ:
I can't attack ___X___ npc!!!
- The NPC is placed in the quest category (assuming you have quest characters protected), or
- It's a bug that you've found.

Can I dismember/Loot Guards?
- Yes, you can dismember. And some guards even drop loot (shoves copy of Blood & Wine under bed)

I have the Break the Game Edition, why can't I kill Zoltan?
- Zoltan has a lot of health, you just need to hit him many times.