The most famous tyrannicide in history is the killing of Caesar in the Senate at the hands of Marcus Brutus. Basically it is the killing of a tyrant in the name of the public good and to restore the democratic (or oligarchic) mores of state.

Most legal systems are premised upon the state having a monopoly on violence. Where any private violence is subject to the review of the state to determine its validity. Tyrannicide is outside of that system. It is based on the assumption that the state is no longer valid, usurped by the tyrant, and consequently tyrannicide is legitimate private violence against the tyrant. Lintott writes of Cicero's view on tyrannicide:

For us the disquieting features of this view are first the idea that a tyrant has no rights at all and no claim to justice, and secondly the extension of the principle to quasi-tyrants.

The quasi-tyrant in Cicero's view was a demagogue who had a 'lasting hold on the populace' even if the demagogue had not used violence or force. The other side of Cicero's view of quasi-tyranny was the use of limited violence outside of the state legal system in order to save and restore the state - in other words a classic state of exception.

There were legal positions in the Roman state, such as the Dictator, which existed in states of exception, so such principles were not unknown to the Roman system. It is unsurprising as Roman magistracy was a state based form of pater-familias and absolute power. Lintott continues:

Tyrannicide is therefore a permissible form of private violence (like that employed in defense of a tribune or against a thief) whose justification lies not only in political philosophy but in a specific legal provisio.

Cicero's view of tyrannicide is expansive and enables the defense of the senatorial and equestrian order's power from the plebians and their demagogic populaire leaders. Consequently private violence can be used to maintain the status quo of state; whether in the death of a dictator-for-life tyrant like Caesar or populists like the Gracchi.

This brings the use of private violence into the domain of politics. And like the state of exception becomes the line where politics and the judicial intersect. The use of violence for justice and redress becomes purely political.
One of the results of an exception being created is that the politics become unitary. Essentially the politics around the exception or emergency become the executive and executive's alone. The health of liberal democracy is dependent upon political competition, discussion and deliberation. Removed of its liberal component democracy is reduced to the mechanical action of voting. (more)
Rome did not have a written constitution as the United States or Australia do. It did not even appear to have a Westminster style one such as Britain's or Tasmania's which exists across multiple non-contiguous acts. It seems to be purely a mix of convention and tradition. Which was probably why it was easy for Sulla, Caesar and Augustus to subvert it. (more)

Hobbes in Leviathan calls the liberty of an individual under the sovereign, and free from the tyranny of all against all violence as an artificial commonwealth man. In modern language we would probably call it political rights. It is interesting to compare Hobbes' artificial man with Agamben's Homo Sacer , in Hobbesian terms, homo sacer is a man entirely under the sovereign yet denied the just bonds of the artificial commonwealth man. (more)

John C. Halez has a thought provoking article on Gary's site where he writes: "no legal system is autonomously self-subsistent and self-regulating, but rather all legal systems will contain areas of indeterminacy, unpredictable and depending on historical circumstances and conditions, which must be 'supplemented' by political decisions." (more)

Giorgio Agamben's thesis in his books have been that the state of exception has become a common form of governance to get around constitutionalism. The most recent excuse for governing under a state of emergency has been terrorism. This subversive form of governance has not been limited to national or state governments. (more)

The meaning of the word security has changed in the last decade or so. Where once it meant stability in defence from the Hobbesian nature of international relations; it has been turned inward to focus on domestic security. So much so that recent op-eds in the Washington Post have made the claim that a city that is not secure - is a failed one. Where once war was deemed an emergency period, with terrorism, Governments have claimed a permanent domestic emergency. This is at odds with Australian Republicanism. (more)

From Multitude;

The idea of Republican virtue has from its beginning been aimed against the notion that the ruler, or indeed anyone, stands above the law. Such exception is the basis for tyranny and makes impossible the realisation of freedom, equality and democracy.

I would add prosperity to this list too. (more)
Cam Riley: South Sea Republic. Freedom, liberty, equity and an Australian Republic.