Diana was damaged let's face it, her parents had a terrible divorce when she was what, 5? Then all the girls in the family being overshadowed by the birth of the heir, Charles Spencer. Then boarding school, where she struggled.
And then she got even more damaged by going into marriage with an older man who didnt love her, they had no shared interests, and he was still in love ("whatever that means") with Camilla. So damaged child lashes out left right and centre, the Firm get all arsey, and it's clear there's no easy way out for any of them.
She then produced the heir and the spare, so there's no further use for her. I reckon she just got really pissed off - a combination of the Spencers being more aristocratic than the RF and the attention of being PoW that went to her head - and it all spiralled downwards from there.
No one is innocent in this scenario, but at least Charles only pursued one woman (two if you include Kanga who seems to have been more of a FWB) while Diana charged her way through several other people's marriages...