Tyrannicide, similar to regicide, literally means the killing of a tyrant, and usually is taken to mean the killing of a tyrant by private person(s) for the common good.
There are many examples of tyrannicide in history, such as Marcus Junius Brutus and Cassius murdering Julius Caesar, the Russian Revolution, and many attempts to take down Fidel Castro of Cuba. The United States attempt to remove Mohamed Farrah Aidid of Somalia from power can be seen as a form ot tyrannicide.