Fairly, it's quite obvious to all that Charles and Diana were terribly suited to each other and both were flawed human beings who treated each other badly.
Most of us have been in bad relationships where things deteriorate to a point in which you bring out the worst of each other. Most of us have lashed out and said or done things we regret when deeply hurt, frustrated or angry. I personally don't think we should judge either too harshly for their marriage and think the fault was equally shared.
Furthermore, yes Diana was terribly young but she was also warned by her family against marrying Charles, who pointed out how poorly suited they were and believed she'd be miserable. Like others have said, she wasn't forced down the aisle.
To the credit of the Royal Family, they learned from their mistakes and changed their entire approach in welcoming women marrying Princes.
And at this rate, I sound like a bloody monarchist, which I'm not..!