The craven need for other explanations than "sometimes, terrible people do terrible things" leads to a lot of undeserved stigmatization.
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Understanding and empathy for dangerous people is one thing, and often brave.
Conveniently explaining away all despicable behaviour as illness, well, that's cowardly.
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We've all been formed by the same primordial soup. Some turn out worse for it. C'est la vie.
It doesn't mean they're necessarily ill.
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And if they are, that doesn't mean this sufficiently explains the fucked things they do.