The prime minister could be forgiven if he looks at recent byelections and the performance of the leaders aligned against him and feels as unbeatable as Achilles.But his vulnerable heel has always been the sense of entitlement that is only occasionally on public display.With the release of the investigation by the ethics commissioner into Justin Trudeau's Christmas vacation on an island in the Bahamas owned by the Aga Khan, there it was for all to see — and it has pierced his impregnability like a poisoned arrow.