I'm listening to a podcast from Relevant Radio talking about how many people in the USA think it's OK to assassinate politicians. Wondering what that Natural Law (as interpreted by the Catholic Church) would say about that. Chesterton quipped once that it was distressing to think of how few politicians are hanged these days, but I think he may have said that in jest. Today people seem to take that seriously. Is there any area where assassination is justified? In a war perhaps?
According to the Catholic Church, is assassinating politicians ever justifiable under the Natural Law?
The short answer seems to be in the affirmative, as long as it was not done by a individual on his own initiative. To truly be assassinated according to this thought, the person or persons must have acted on the moral decisions of those who have the moral competence and authority to make the call. To act on one’s own initiative would be considered murder and is a sin according to the natural law.
St. Thomas Aquinas in his Summa Theologiae has this to say about private individuals killing someone
As stated above (Article 2), it is lawful to kill an evildoer in so far as it is directed to the welfare of the whole community, so that it belongs to him alone who has charge of the community's welfare. Thus it belongs to a physician to cut off a decayed limb, when he has been entrusted with the care of the health of the whole body. Now the care of the common good is entrusted to persons of rank having public authority: wherefore they alone, and not private individuals, can lawfully put evildoers to death.
It seems that assassination may be morally permissible under certain circumstances.
Yes, in a time of war, for example. Pope Pius XII supported efforts to assassinate Adolf Hitler during World War II. Scholar Mark Riebling substantiated this in his important book Church of Spies: The Pope’s Secret War Against Hitler. And Riebling’s book, as well as the efforts of others, have led many critics to change their minds about Pius XII.
However, the Church’s “just war” principles would not permit a preemptive assassination of a leader such as Kim Jong Un (CCC 2309). However, actions to impede his launching nuclear weapons would be morally permissible (see CCC 2310ff.).
The one to be assassinated must be clearly proven of criminality, such as tyranny.
The moral issue here is that of tyrannicide the killing of a tyrant, and specifically, the killing of a tyrant by a private person for the common good. Technically, there are two classes of tyrants: a tyrant by usurpation (tyrannus in titulo), a ruler who has illegitimately seized power; and a tyrant by oppression (tyrannus in regimine), a ruler who wields power unjustly, oppressively, and arbitrarily.
Tyrannicide has had support from various philosophers and theologians through the centuries, including the ancient Greeks and Romans, most notably Cicero; Catholics, most notably John of Salisbury (d. 1180) Jean Petit (d. 1411), and Suarez (d. 1617); and Protestants, most notably, Luther, Melanchthon, Zwingli, and Calvin.
St. Thomas Aquinas gave the most substantial argument for tyrannicide. He based his position on his arguments for just war and capital punishment. St. Thomas concluded, "He who kills a tyrant (i.e. an usurper) to free his country is praised and rewarded" (In 2 Sentences, 44.2.2).
A tyrant by usurpation has illegitimately seized power and, therefore, is a criminal. When there are no other means available of ridding the community of the tyrant, the community may kill him. According to St. Thomas, the legitimate authority may condemn him to death using the normal course of law. However, if the normal course of law is not available (due to the actions of the tyrant), then the legitimate authority can proceed "informally" to condemn the tyrant and even grant individuals a mandate to execute the tyrant. A private citizen who takes the life of a tyrant acts with public authority in the same way that a soldier does in war.
The key conditions for a justifiable act of tyrannicide in this case include that the killing be necessary to end the usurpation and restore legitimate authority; that there is no higher authority available that is able and willing to depose the usurper; and that there is no probability that the tyrannicide will result in even greater evil than allowing the usurper to remain in power. - Does the Church Condone Tyrannicide?
The Spanish scholastic Juan De Mariana, S.J. (1536-1624) defended tyrannicide, that tyrants can be assassinated.
But [Fr. Juan de] Mariana insists on another very important idea. He says natural law is vastly superior to the power of each king or ruler. This is an essential idea that is still perfectly applicable today. I believe that we must be aware of and very proud of the fact that when Thomas Jefferson, the founding father of the United States, was considering whether or not to rise up against the king of England, in order to give encouragement to Madison and the rest of the founding fathers of the great American homeland, he says to them: "You only have to read one book: The History of Spain by Fr. Juan de Mariana." And, in fact, Madison even sent him a copy of the book*, because it is a history of Spain written from the perspective of freedom, unmasking the tyrants.
*[the "12mo" or duodecimo version of vol. 2; this is the scan of Madison's copy from Internet Archive]
The Dominican lay brother Jacques Clément, O.P. (1567–1589) assassinated King Henry III of France*, and Pope Sixtus V praised him†:
[A] work famous, memorable, and almost incredible, a work not wrought without the special providence and government of the almighty; a Monk hath slain a King, not a painted King, one figured out upon a piece of paper or upon a wall, but the King of France, in the middle of his army, being hedged in with his camp and guard on every side, which indeed is such a work, and so brought about as no man will believe it when it shall be reported, and the posterity perhaps will repute it for a fable.
—Consistorial address of the Supreme Pontiff Sixtus V in praise of the assassination of King Henry III of France by Br. Jacques Clement OP.