I agree that the whole thing was a mess and a disaster waiting to happen. But I don't like assigning guilt precisely because it was so complex.
Charles had been given to understand that he must marry a virgin. He fell in love with a couple of women (at least!) before his marriage, but they were not 'acceptable' as potential future queens. I remember that, as well as other well-known girlfriends mentioned here, he was particularly fond of Davina Sheffield but - horrors! - she'd had a previous boyfriend. Thank goodness things have moved on since then.
Diana was a lamb to the slaughter as a 19-year old ingenue with a difficult family background and obvious insecurities. But the Spencer family (her grandmother in particular) were 'guilty' too, because they very much manoeuvred her into the marriage.
Both Charles and Diana had their psychological issues (he a bullied and lonely child/teenager, she growing up without the presence of her mother etc) and, with the pressure and scrutiny they both came under, I just don't think it could ever have gone well.