Setting the referendum up as advisory and non-binding, but then treating it as though it was definitive and legally binding - that is on Cameron.
Setting it up so as to necessitate a huge, complex constitutional change based on a simple 50%+1 majority, rather than a 2/3 supermajority, as almost every other country requires for such a significant constitutional change, that is on Cameron.
Making absolutely no preparation for what happens in the event that things dont go according to plan, plunging the country into chaos as soon as the vote was announced, is on Cameron.
Calling a referendum in the first place, putting a ridiculously complicated and nuanced question that most MPs dont even understand, let alone the public, to be decided as a simple, binary question, all to appease a minority of bellends in his own party, is on Cameron.
Cameron is far more culpable for the mess we are currently in that May IMO.