As far as I know (not legal advice), the U.S. has extraterritorial laws that allow it to prosecute the murder of American citizens anywhere in the world.
If the foreign country has an extradition treaty with the U.S., this individual can be brought to justice peacefully. If he takes refuge in a country like Libya with no extradition treaty, I think our government should just have him killed; Israel has done this to terrorists outside its territory on numerous occasions...
well there exists nothing like international laws that would allow the US to prosecute the murder of American citizens anywhere in the world. so if you do it (and this is a given fact that not only the US does something like that in this world it´s still illegal. but here comes the important part. You like every other country does this only when the hunted person is spotted in a country “you” have a very bad relation anyway (so it can´t get more worst) or to poor backward countries where you know even it became public no one will especially care because of that nor will it have serious consequences. there are as you said extradition treaty between different countries. but there for you need a total legal basis that they will hand the guy over. so as long this doesn´t exists there is not much you can do. yes you are correct for example the Israelis did this. there are still mossad agents rotting in a norwegian prison (setenced long ago for lifelong prison because of murder) because they did this and have been catched. (let alone the harm this did for the relations between “europe” and Israel). they haven´t tried something like this (or at least have not been catched) in europe again since then, guess why? so it´s not that simple. for example the US had to guarantee germany that one of the terrorists they catched (responsible for 9/11) can not be setenced to death (because it´s against the law in the EU to extradite some one who could face death penatlty) in a us court. Only after the US guaranteed this the germans handed the terrorist over. (they would have been forced to set him free if the US had not done this). Isn´t international politics fun? ;-)
greetings
Re: “for example the US had to guarantee germany that one of the terrorists they catched (responsible for 9/11) can not be setenced to death (because it´s against the law in the EU to extradite some one who could face death penatlty) in a us court. Only after the US guaranteed this the germans handed the terrorist over.”
I think most families of the victims would settle for this terrorist getting life without parole in a Supermax prison, especially if the prison sent them a photo of what the typical maximum security cell looks like.