This is just my own opinion.
I think there's the same co-dependency dynamics going on in this mother and daughter relationship as there would be in other kinds of abusive relationships. It takes therapy for both because most times one is the enabler allowing the behavior to continue for reasons that need to be examined.
Example - a spoiled child needs someone to spoil them. They don't get
there on their own.
As I said, JMO.
It's an incredibly sad situation. It seems that Heather lacked the discipline she needed, which resulted in increasingly out of control behavior. Her mother tried to give her a luxurious life, but it seems that Heather wanted or needed something different. She must have believed that if she murdered her mother, she would get away with it, and then she would have freedom, all the money, and that she could live the reckless life she wanted. If she lives to the age of 25 or 30, she might be able to understand what she did.