Somebody in our Diplomatic corps screwed up ROYALLY on this. This is not a reason you choose to extradite a US Citizen(s) to a foreign country.
If that is the case, why should we expect a foreign government to extradite one of their citizens who commits a crime in the US (or elsewhere but still under US jurisdiction), and then returns home to their country?
If the tables were turned, and a mexican national came into the US to catpure and bring back a mexican rapist who was here for whatever reason, would we be upset? Questions about the efficiency etc. of the US vs. mexican law enforcement/judicial process aside, the answer would be a resounding YES!!
Whether or not a foreign government cooperates with US law enforcement in attempts to aprehend a US national outside of the US does not entitle anyone (especially one who proports to uphold the law) to take matters into their own hands.
In short, no one is above the law, in this country or elsewhere. Certainly not those who are supposed to be enforcing the law.