Diana wasn't mad. She was just too young to marry Charles (look at William and Kate, they are 25 and after 4 years they broke up ...). I think she was waiting for so much affection (maybe because her mother didn't really gave it to her ...). Charles, who was toughly educated, simply didn't knew how to comfort her and show the affection she wanted. In a relationship, both are "responsible" for the sadness they cause to each other. Maybe it was an act of despair or a simple accident, we will never know. In any case, Charles couldn't hate Diana, his wife, at a point to stay indifferent to her sadness. IMO he really tried to make her happy and Diana tried to BE happy but what they endured in their childhood (for Diana a mother a little alcoholic and for Charles a very strict education) may have touched them more deeply than they thought.