When a political leader is assassinated, such as Benazir Bhutto yesterday, the assassins usually hope to influence the future direction of the society they reside in. There is an argument to say that almost all assassinations fail to achieve the intended result, with the results often being exactly the opposite of what the killers intended. A recent example of that tendency would include the assassination of Pim Fortuyn, after which having demonised him so thoroughly in life by making out that he was some kind of Jorg Haider or Jean Marie Le Pen figure the main Dutch political parties have all adopted significant aspects of his platform. This kind of result is quite common in stable countries where assassins tend to simply make martyrs of the victims whose successors can pursue the same policies with greater vigour as a result of the ensuing public sympathies.
The problem with the Bhutto killing though is that she did not run her party as a constitutional organisation with defined rules of succession, rather it is a dynastic organisation where the power was far too dependent on one person whose death has the potential to create a power struggle that will negate any sympathy votes the party will receive.
Still the list of assassinations that were successful in changing history to the murderers liking is rather short, the assassinations of Admiral Carrero Blanco and Leon Trotsky spring to mind as isolated examples but those are very much the exceptions.