Apparently he's worth north of £100m, so I can't see money being his motivation here. More likely, it's revenge against the person who, in his view, more than anyone else was responsible for the death of his sister.
Whoever he is, and whatever his faults may be, is largely irrelevant to the point. He's neither the head of state nor someone living off the public purse. He's simply another aristocrat with a very personal grievance.
And I can see no obvious reason why he would lie about what happened. The bare bones of what we already know appear to give at least some credibility to his account. That doesn't mean every allegation he makes is necessarily true, but it does mean his version shouldn't simply be dismissed out of hand.
What can't be avoided, at least among some royalists, is the uncomfortable sight of a figure who has been worshipped by some being revealed as an intemperate , vicious angry man with the emotional intelligence of a rock.