This mod from Zumbs is pretty self-explanatory. It allows you to kill NPC's that are marked as essential.
Readme File:
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Kill and Loot Unconscious Essential NPCs
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Version: 1.0
By Zumbs (zumbs at yahoo dot com)
November 11th 2009
1. Introduction
2. Requirements
3. Installation and uninstallation
4. Compatibility
5. Version history
6. Terms of Usage
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1. INTRODUCTION
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After activating this mod you will be able to loot and kill unconscious essential NPCs (and creatures) by activating them. Only the player is able to kill essential NPCs, so it will not accidentally damage your game. To get the kill option, you need to have a weapon equipped.
Note that killing an essential NPC or creature may make quests malfunction, so you kill them at your own risk.
Due to limitations in the scripting engine, only NPCs and creatures made essential by unmodded Fallout 3 are affected by the kill option. The DLC are no supported as I do not own them.
As this mod works by adding a perk to the player, it is highly recommended that you apply the latest patch to your game.
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2. REQUIREMENTS
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Fallout 3 version 1.7.0.3
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3. INSTALLATION AND UNINSTALLATION
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a. Install
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FOMM
1) Open the package manager
2) Press Add New and browse to where you saved this archive
3) Highlight the mod and press Activate.
Manual
1) Extract "Kill and Loot Unconscious Essential NPCs.esp" to the Fallout 3/Data folder
2) Open your prefered mod manager and activate the mod
b. Uninstall
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WARNING: If you have not applied the 1.6 or later patches, you will need to remove the perk before uninstalling. Open the console and write
startquest ZumbsLootKillPerkAdder
Then follow the steps below
FOMM
1) Open the package manager
2) Highlight the mod and press Deactivate
Manual
1) Open your favorite mod manager
2) Uncheck "Kill and Loot Unconscious Essential NPCs.esp"
3) Delete the files
Kill and Loot Unconscious Essential NPCs.esp
Kill and Loot Unconscious Essential NPCs - readme.txt
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4. COMPATIBILITY
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This mod can be placed anywhere in the load order, and should not conflict with anything. Killing essential NPCs may screw up your game, but no one is forcing your hand!
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5. VERSION HISTORY
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Version 1.0: Initial release (November 11th 2009)
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6. TERMS OF USAGE
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You are free to use, distribute and edit this mod. If you do either of these things, I ask that you kindly notify me.