If we have to execute prisoners, why don't we do it with a bullet to the back of the head instead of lethal injections?

Now first of all, I'm against the death penalty but here's what I don't get: if we have to execute people, why do we use complicated execution methods like the electric chair and lethal injections? Why don't we just use a gunshot to the back of the head?

If we have to execute prisoners, why don't we do it with a bullet to the back of the head instead of lethal injections?
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