DO NOT USE THIS WITH BETTER SETTLERS - THEY ARE INCOMPATIBLE - USE THE "MORTAL" PATCH THERE INSTEAD.

This mod is for vanilla settlers only.  Please see my mod Better Settlers for some souped up settlers that will be more able to handle Mortality.


Changelog is at the bottom of this post.  Current version: 0.7

Right now, generic settlers are protected, which means they can only be killed by you.  They are basically immortal when it comes to the NPC attacks on your settlements.  They enter bleedout for 15 seconds, then get back up and will repeat this cycle until all attackers are dead.  So there is no point in you even helping with defense or giving them good equipment as they will eventually kill everything, zombie style.  I did not like that, so I removed the protected flag from MOST of them.  Companions, Provisioners, Children, and Named settlers are unchanged, and will behave as in vanilla (mostly protected).

If it matters, I made it with FO4Edit.

What does this mod do?

Removes the protected flag from most settlers so they can now be killed in the settlement attacks by NPCs.
The "starter" settlers (Sturgis and the Longs) have not been changed, so it doesn't break your game.
The "Provisioner" settlers that travel between settlements have not been changed, so it doesn't break your game.
Companions have not been changed, so it doesn't break your game.
Children have not been changed, and will behave as in vanilla (protected I think).
"Named" Settlers have not been changed, and will behave as vanilla. (some are protected Like the Finch, Abernathy and Warwick family - some are not protected, like the random named guys at Warwick farm).

What is the effect?

You should actively defend your settlements.  
You should equip them with good equipment.
I would hesitate to give them grenades, missiles, molotovs or fat man because they are flat-out psycho with them.

How did you test it?

Loaded mod, went to Sanctuary, Abernathy Farm, Finch Farm, and Warwick Farm set myself to god mode, spawned a lot of raiders and watched.  
Did this 3 x.
Provisioners and the Longs entered bleedout like normal and got back up.
Companions (Codsworth) entered bleedout like normal and got back up.
The principal named settlers entered bleedout like normal and got back up.
Generic Settlers died.

What are the known issues?

Known Issue:  I believe the sole know issue has been fixed.  They should no longer get crippled, and they should now heal between battles.  Please let me know if this does not work.

There is some game setting that despawns corpses on some condition.  I dont know what it is.
While I was walking through the bodies, a settler body despawned, so whatever she had equipped was lost.
So you might lose items on corpses if they despawn. (If someone tells me how to fix it, I will).
I am a very novice modder and am no expert.  
THIS MIGHT BREAK YOUR GAME EVEN THOUGH I TESTED IT.BE WARNED.

BE WARNED.  Although I will feel terrible, I take no responsibility if this breaks your game.  BE WARNED.

If you have bugs, fixes, suggestions, etc, please post comments and I will try to address.

Installation
Make sure you have made all the necessary configurations to enable mods in Fallout 4.
If you haven't, check out this guide to learn how: FALLOUT 4 - Enabling Modding for PC by Gopher.

I seriously recommend installing through the Nexus Mod Manager. The installation wizard guides you through the steps, and it minimizes the potential for future errors. 

With Nexus Mod Manager (NMM):
1. Download the mod by clicking "Download with manager".
2. Activate the mod in Nexus Mod Manager

Uninstallation
With Nexus Mod Manager (NMM):
1. Deactivate the mod by double-clicking it.
2. Mark the deactived mod and uninstall or delete it.

CREDITS:
drdanzel - I copied his installation instruction section!

Changelog Version History:
Version 0.7Recent Changelog 0.7 
Gave settlers "Lifegiver 3" and "Adamantium Skeleton 3" perks so they cannot be crippled and so they will heal between battles. 

0.6 - Added Readme.

0.5 - Initial Release.