Preventive tyrannicide is killing a ruler for the evil he may do, not for the evil he has already done. It is acting in advance, against a tyranny that has not yet happened. This is the dilemma that underpins every scene of Shakespeare's Julius Caesar, and it is a problem that did not die in 44 BC.
Kill what doesn't yet exist
Caesar, in the play, is not a tyrant. He won the civil war, forgave his enemies and was acclaimed by the people. The conspiracy acts against what he can become. Brutus formulates the logic in an exact image: Caesar is like the serpent's egg, which "must be killed while still in the shell", before it becomes poisonous.
Therein lies the crucial difference. Classic tyrannicide responds to a crime committed: the tyrant is overthrown for the evil he does. Preventive tyrannicide responds to a prediction: someone is eliminated because of the evil they are feared to do. One judges an act; the other, a presumed intention.
Why the problem is insoluble
The ethical difficulty is that a prediction can be wrong, and there is no way to confirm it without letting the future happen, which is precisely what we want to prevent. To what extent does the fear of possible evil justify acting violently against it now? Think about a decision you made out of fear of what “could” happen, rather than what actually happened. Was it prudence, or was it excess?
Brutus' remedy does not prevent tyranny: it produces civil war, proscriptions and the rise of Octavius, exactly the empire he wanted to avoid.
The answer that history gives
Shakespeare does not close the issue in speech, he closes it in the plot. Caesar's murder does not save the Republic: it opens a civil war that kills one hundred senators, including Cicero, and ends with power concentrated in the hands of Octavius, the first emperor. Brutus wanted to stop a man who was too strong; helped create the Empire.
It is the harshest lesson of the play: moral purity without prudence is catastrophic. Every time someone acts today to prevent an evil that only exists as a possibility, this same logic is at play, and Shakespeare himself leaves the question without an easy answer. The question remains yours: is it legitimate to kill to stop an evil that does not yet exist?
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What is preventive tyrannicide?
It is the murder of a ruler not for the evil he has already done, but for the evil he is feared to do. We act in advance, against a tyranny that is still only a possibility.
What is the ethical problem with preventive tyrannicide?
That he punishes a presumed intention, not an act. It is based on a prediction of the future that may be wrong, and, when using violence against an evil that does not yet exist, it usually produces exactly the evil that it wanted to avoid.
How does Julius Caesar deal with this topic?
Brutus kills Caesar for what he could become, not for what he did: the serpent's egg that must be crushed in its shell. The result belies the bet: crime does not prevent tyranny, it generates civil war and the Empire.
Go deeper: Julius Caesar, by Shakespeare: summary and analysis · Who was Julius Caesar · Stoicism and Epicureanism: the difference
Source class (YouTube): Júlio César, de Shakespeare (NousCast)