She had a medical condition that caused it. Just because you've never had such a condition doesn't mean it doesn't exist. The world is bigger than your experience. If you can imagine yourself having a heart attack forcing your body to stop driving and kill your kids, why can't you imagine yourself with a mental illness that forces you to do terrible things?

No. That's entirely not the point. If you have a heart attack and so your body forces you to fall unconscious and so you get in an accident and kill your children, are you guilty? Obviously you need medical attention and cannot be allowed to drive while there is any significant chance that you might do it again.
Nobody is suggesting that this woman should be set free - she needs to be in a secure mental treatment facility until she is safe to be in society - and possibly that will never happen.
This has nothing to do with those other issues or skin color. It is about mental health, women's support and post partum care.
If you can't wrap your head around the mental illness cause part because you have no empathy or ability to understand things you haven't experienced, maybe think about how one's mind can be altered with drugs/alchohol, or how hypnotism can make people do complex weird things. Our brains are a very complex chemical system and can get out of whack causing surprising results. Post partum mental illness is much more common than most people believe because of the stigma and covering it up. It needs a lot more understanding and accommodation - that recognition and support would help avoid such tragedies as this case. Your inability to understand and empathize will only make things worse.