In my experience, normally it's when they think they'll gain an advantage from it - one that can't be had by any less extreme route. It's the same as saying "all murderers are mentally ill, because only a mentally ill person would want to kill - so let's be nice to them, they're obviously suffering." Nice in theory, in practice likely to cause a real mess. And obviously it ignores the majority of people who are mentally ill and don't harm a fly, even those who struggle to overcome violent urges at times. JMO.