I mean aside from the usual idiots: Cameron, Johnson, British geezers..., etc. Kevin Drum takes a look at a long list of suspects.
Here he is on Angela Merkel:
For all the praise she gets, Angela Merkel has been one of the most disastrous European leaders in my lifetime. She's as responsible for Brexit as anyone I can think of, thanks to two catastrophic decisions she made.
The first was her insistence on punishing Greece following its collapse after the Great Recession. There's plenty of blame to go around on all sides for the Greece debacle, but as the continent's economic leader Germany held most of the high cards during negotiations over Greece's fate. Merkel had a choice: (a) punish Greece for running up unsustainable debts and lying about them, or (b) accept that Germany bore much of the blame itself for the crisis and that Greece had no way of rescuing itself thanks to the straitjacket of the common currency. The former was a crowd pleaser. The latter was unpopular and would have required sustained, iron-spined leadership. In the event, Merkel chose to play to the crowds, and Greece has been a basket case ever since—with no end in sight. It hardly went unnoticed in Britain how Europe treated a country that was too entangled with the EU to either fight back or exit, and it made Britain's decision to forego the common currency look prescient. And if that had been a good choice, maybe all the rest of "ever closer union" wasn't such a great idea either.
Merkel's second bad decision was more recent. Here is David Frum: "If any one person drove the United Kingdom out of the European Union, it was Angela Merkel, and her impulsive solo decision in the summer of 2015 to throw open Germany—and then all Europe—to 1.1 million Middle Eastern and North African migrants, with uncountable millions more to come." It's hard to fault Merkel for this on a humanitarian basis, but on a political basis it was a disaster. The barely-controlled wave of refugees Merkel encouraged has caused resentment and more all over Europe, and it unquestionably played a big role in the immigrant backlash in Britain that powered the Leave vote.