In “Hit or Miss? The Effect of Assassinations on Institutions and War,” Olken and Jones looked at the effects of political assassination, using a strict empirical methodology that takes into account economic conditions at the time of the killing and what Olken calls a “novel data set” of assassination attempts, successful and unsuccessful, between 1875 and 2004.
Olken and Jones discovered that a country was “more likely to see democratization following the assassination of an autocratic leader,” but found no substantial “effect following assassinations—or assassination attempts—on democratic leaders.” They concluded that “on average, successful assassinations of autocrats produce sustained moves toward democracy.”
From a profile of Ben Olken in the American. And here's the paper (free access).