[Piers]

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Will it dissuade others from committing murder?

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Rarely.

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Is there more than a micro chance that the life sentence will allow an eventual release or escape?

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Irrelevant.

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Does it cost more to keep him alive?

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Obviously. Certainly this argument has been used before to kill people.

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Is there more than a micro chance that the defendent is innocent?

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Usually.

I don’t think the OP’s four points are relevant. Killing criminals is all about public revenge, its up to society decide what it wants. Personally I don’t care much one way or the other.