Why not use lethal means?

Why would a person choose to defend himself or herself (self-defense) with non-lethal means?. It would seem to me if I had a need to defend myself against harm, lethal means would be the most effective way.
Self-defense...is a countermeasure that involves defending the well-being of oneself ...from harm. http://ift.tt/1IQmEBM
harm -

noun
physical injury, especially that which is deliberately inflicted.
synonyms: injury, hurt, pain, trauma

verb
physically injure.
synonyms: injure, hurt, wound, lay a finger on, maltreat, mistreat, misuse, ill-treat, ill-use, abuse, molest
http://ift.tt/1QMBLRx

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Why not use lethal means?