Killing him while being effective is giving him the easy way out, one that is quick and with the minmum of suffering. Where as life inprisonment at hard labour will give him the chance to suffer extreme harshness as well as contribute to rebuild some of the damage he has done. Also with the ego he has the humiliation he will expierience at having to do such demeaning manual labour will double the efffect of the punishment. And lets face it for scum like him a bullet or rope is too good for them.

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What is life?
If not an excuse for death,
and what is death,
if not an escape from life?