The issue surely is not about whether DC paid the appropriate taxes when selling his shares. It is about a disconnect between what he says in public and what he does in private, and about what could be considered to be a conflict of interest in that he made representations to the EU to protect offshore trusts.
Accountancy Age, and other media outlets, reported a few days ago:
"DAVID CAMERON continues to be the subject of extreme criticism regarding the Panama Leaks, after it was discovered that the prime minister went out of his way to protect offshore trusts from EU tax avoidance measures.
"........... An apparent supporter of tax transparency, in 2013 Cameron wrote to Herman Van Rompuy, European council president at the time, arguing that offshore trusts should be exempt from an EU clampdown on money laundering, creating a loophole that could be exploited by tax evaders."
Accountancy Age further reported on 11 April:
"According to the Financial Times, Blairmore issued an investment prospectus in 2006 making clear that the fund would not be subject to UK tax on its profits. It said: “The directors intend that the affairs of the fund should be managed and conducted so that it does not become resident in the UK for UK taxation purposes.”
In my view, it is quite possible to do something that many people would consider to be "wrong" without it being illegal. Making pronouncements as to the morality of others while not being exactly squeaky clean yourself, and making official representations on behalf of your country about issues in which you may be considered to have a personal interest may not necessarily be punishable by law but it would still be assumed by many people to be "wrong".