The news that former Japanese Prime Minister Shinzo Abe was assassinated Friday, at the age of 67, during a campaign rally came like a thunderbolt. If it was shocking to people around the world, it was devastating in Japan, where gun violence is essentially non-existent. Abe’s killing would have been appalling at any time. Now, however, it adds to the sense of an unstable world in crisis — in which democracies, in particular, appear to be under siege.