Three... Diana was no angel either
Honestly? I never thought I'd ever articulate anything like this, but she was a very immature, naive, and barely educated young woman - thick as two short planks, by her own admission an 'airhead'.
Albeit very pretty and with a good helping of ingenue charm, she was superficial and with no other purpose than to marry the dashing knight on the white charger, per her step-grandmother's trashy romance novels which she consumed voraciously, and automatically live happily ever after. Worse than that, she came from a very abusive home and her own grandmother, Chuck's grandmother and Dickie Mountbatten engineered her into this disastrous wedding.
As I see it now, she acted out her anguish at the crap marriage she'd entered into. Just like her younger son, she lacked the capability to work things out in a more sensible, more mature way. Like her son, she did a lot of damage in her acting out, and ultimately damaged herself fatally.
Chuck should have been far more responsible. For all his learning and philosophising he picked someone he vaguely imagined (or had been led to believe by his Uncle Dickie) that he could, disrespectfully, bend to his will.
13 meetings/'dates' before the wedding?! How insanely irresponsible is that?!
It's all very tragic.