I think I struggle with her because usually, these cases are wrapped up in the struggles of poverty, addiction, domestic abuse etc. Which doesn't excuse their crimes, but do make it easier to understand how it happened.
With Marten... So many of the barriers and struggles experienced even by your average middle class family just... didn't exist for her. If she wanted a house or a car, it would have been bought for her. If she didn't want to work, she didn't have to, she had a middle class family's income as pocket money. Private therapists, bankers, lawyers, healthcare, childcare were always just a phonecall away. Even without it, she had her education, friends and family. Even her upper-class accent and mannerisms are a serious asset in the UK.
Class and money does not invalidate her trauma, mental health, unstable childhood... but she only had to click her fingers for the kind of help/resources ordinary people can only dream of.