It's obviously not an excuse and while it can help to explain her actions, it also shouldn't serve to lessen her punishment at all (and likely won't). All a successful "not guilty by insanity" plea guarantees is that she will serve her sentence in an insane asylum instead of a prison, and that on top of a specific number of years (or life) sentenced, she also won't be able to be released until mentally cleared by a doctor.

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There’s a difference between an excuse and an explanation. Planning something doesn’t mean you’re sound of mind. I don’t think anyone is excusing the murder of 3 children, but it is also extremely relevant to bring up her active psychosis.

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