(1) Assassinations create a groundswell of public support for the assassinated. Look at Julius Caesar, a violent, anti-democratic dictator who was assassinated for the best of reasons. Look at JFK: the Bay of Pigs and getting the US into the Vietnam War were all but forgotten.
Assassinations make martyrs, worthy or unworthy. Julius Caesar's reputation is far better than it should be for a man who committed mass murder routinely, and it is largely due to his assassination.
(2) Assassinations tend to create an opportunity for other would-be dictators to take over. Julius Caesar's assassination didn't lead to the restoration of the Republic; it led to the Second Triumvirate and Emperor Augustus. Modern African countries after decolonization provide hundreds of examples. Assassinations encourage the collapse of democratic structures, even when they're intended to restore them.
(3) Assassinations, and even attempted assassinations, create yet another excuse for attacks on civil liberties.
Political executions are usually nearly as bad, but not always. The execution of Charles I led to Cromwell, and the execution of the French monarchy led to Napoleon. The "execution" of Saddam Hussein made him more popular than he ever was when alive. But there was an execution involved in the Carnation Revolution in Portugal, and that turned out all right. And the executions after the Nuremberg Trials also left Germany OK.
What does work? What should Brutus have done to stop the tyranny (and the collapse of republican government for over fifteen hundred years)?