[PnP] lethality index

[Melkor]

Yesterday i started using this optional rule by Snider.
At times it seemed to me that my players 
Suddenly became overpowered, because one if them already had DTV -4 and so his lethality index resulted -20.
What do you think?
Also, why Snider came up with this rule later but didn't implement it in the original boxed set?
My 2cents is that he realized that P&P is a pretty lethal game...so he thought to give players a chance.

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