Universal jurisdiction, according to Wikipedia, is:
a principle in international law whereby states claim criminal jurisdiction over persons whose alleged crimes were committed outside the boundaries of the prosecuting state, regardless of nationality, country of residence, or any other relation with the prosecuting country. The state backs its claim on the grounds that the crime committed is considered a crime against all, which any state is authorized to punish. (source)
Universal jurisdiction departs from the standard principle that there should be some kind of connection between an act and the state asserting jurisdiction over it. In other words, the normal rule is that states exercise justice in relation to crimes committed on their territory or crimes committed by their nationals abroad. Indeed, this departure is the main criticism of universal jurisdiction: by allowing a state to prosecute individuals who are not its citizens, and who have committed crimes in other states, against people who are citizens of other states, we in fact allow this state to violate the right to self-determination of other states.
However, universal jurisdiction is nothing new, and most countries accept some kinds of universal jurisdiction. For example, few now oppose the right of Israel to judge Adolph Eichmann. The discussion, therefore, centers on the proper extent of universal jurisdiction. Human rights activists claim that states should be able to exercise universal jurisdiction in cases of genocide, crimes against humanity, torture, war crimes and slavery.
The reason behind this list is also the main justification of universal jurisdiction. These crimes affect all of us, the whole of humanity, and not just the immediate victims. Those who commit these offenses are hostis humani generis, enemies of humanity. And this has to be taken literally: these crimes are attacks on humanity, not just on individual human beings. The torturer dehumanizes his victim, but also himself. And he infects the society in which he operates. A society that allows torturers in its midst, can no longer be called a society. The same can be said of genocide and the other crimes in the list.
Universal jurisdiction is the act of reclaiming humanity. It is a statement by different parts of the world community, claiming that humanity does not accept such crimes. It is, therefore, an expression of humanity against those who attack humanity. And it’s a powerful expression of humanity precisely because it emerges from different parts of humanity, different countries and nations which all have an interest in the preservation of humanity.
I can imagine that some would object to all of this and would insist that crimes are committed against individuals, and not against an abstract entity such as “humanity”. But then I would invite those people to explain how they differentiate between a single anti-semitic murder and the holocaust. Or between a single case of an individual torturing another individual, and a case of state organized torture. I do believe that the concept of “crimes against humanity” makes sense, and that universal jurisdiction is a good way to respond to those crimes, maybe not from a purely legal point of view (universal jurisdiction isn’t the most effective jurisdiction) but from a human point of view.