It IS such an unthinkable thing to do, PrimeSuspect. I completely agree. Still trying to understand it:
One of my pet stone-cold cases is the London Torso Murders, which were concurrent with Jack the Ripper (and quite close in proximity as well..). Anyway, I did a lot of research on why people dismember, to get my head around that case.
It basically came down to a couple of very different motives for the act of cutting another person up -- a/ psychological gratification of whatever kind (revenge, rage, paraphilia) and b/ practicality (hiding/disposing of the victim). Not that they're mutually exclusive, but there tends to be different kinds of crimes associated with each. The latter often involves money.
I reckon this woman was likely in the b/category.