Let´s be fair, please. Diana had virtues and failures like any human being.
iI I follow your line of thinking, then a guy like Guy Fawkes should have been spared to pay for his deeds as well?
Diana in her later days, for reasons whatsoever, tried to undermine the monarchy in a way that is covered by the Treason Act - one should not forget about that. Yes, she was only human, yes, she felt she was right in her opinions, but still there is something like laws or at least rules of behaviour out there. Even if this does not mean anything to you, even if you think she was human, so this absolves her - that is not necessarily the view of the monarch whose throne was endangered by a lady who had only married into the RF, who was not of the Blood Royal but who claimed to know much better how to be Royal than all of those born to the Blood Royal.
In the Panorama-interview Diana said that the line of succession doesn't count because of her husband's failures, that only her son, who (she did not say that, but it is implicated, IMHO) despite of his Blood Royal was capable to rule, because he was to be raised by her, the Queen of Hearts and her alone. With that she took the mystique, the sacredness away from the Blood Royal and made the kingship into something anyone could aspire to if he or she was only raised by the right person. Very dangerous thought, this! in addition she gave the cynicism of the media fuel: they have not forgotten that it was them who "discovered" little Shy Di and wrote her into the position of the future queen and mother of a future king. She was their creature and now she ,as an insider, rallied against the monarchy. Okay, for the sake of her own child, but in the mind of the media this was not interesting - it was the fact alone that a figurehead of public connection turned against the monarchy who gave them all the reasons to do the same and feel justified.
If behaving in such a politically destructive manner is only considered a "failure" and "human", so understandable and condonable, then I wonder why society needs so many rules, so many laws to keep it going in a civilised manner. Society needs rules as mankind are dangerous creatures. I have no problem with Diana being unhappy with Charles and wanting out. But I really have a problem with her crucifiying all in order to achieve her goals.
And all the queen did after Diana's death IMHO was to pacify the dangerous creatures called humans that Diana had courted. She succeeded. That's all that counts.