During the Warring States Era all that ended. Something on the order of hundreds of thousands of troops were fielded in battle. These troops were untrained peasants and had a difficult time killing. Maimed and crippled survivors across the region became dependent on their families for basic care.
In response to this problem easy to remember death points were invented and taught widely. As a way to make sure that the suffering on the battlefield stayed on the battlefield; soldiers were offered a quick compassionate death.
(I have no footnotes to support my theory but I do have this wonderful chapter of the Daodejing which mentions death points. It is also the chapter which uses the idea of 'Nourishing Life" (Yangsheng), a term which was later used to describe macro-biotic, hygiene, gongfu, nutrition and other longevity practices-- a core concept of Daoism and Chinese culture.)
Chapter 50 (Daodejing)
Coming into life and entering death,
The followers of life are three in ten.
The followers of death are three in ten.
Those whose life activity is their death ground are three in ten.
Why is this?
Because they live life grasping for its rich taste.
Now I have heard that those who are expert in nourishing life*
Can travel the land without meeting tigers and rhinos,
Can enter battle without being wounded.
The rhino has no place to plant its horn,
The tiger has no place to place its claws,
Weapons find no place to receive their sharp edges.
Because he has no death-point.*
*[The translator, Charles Muller, chose to translate shesheng as "handling life" and sidi as "death-ground." I took the liberty of making these small improvements. Also I should point out that he uses the expression "rich taste" in line 6, where the text has a single term which means: Thick. ]
Update: I just wanted to clarify that the image above is of acu-moxa points not death points. Originally there were no more than 18 death points and 10 was probably enough. The weapons were changing too at that time but imagine getting hit in one of the death points with one of these jade axes!