Andy,
An assasination is what happened to Martin Luther King or JFK. Osama was a TERRORIST.

'Assassination' depends on the person, not what they did, PW  The dictionary definition of the word is:

the murder of a famous or important person, especially for political reasons

(Macmillan Dictionary Online)

Assassinations are usually 'sponsored' by organisations or governing authorities.

Those verses do not tell it only applies within one states borders. Did I miss it?

Yes, you did.  The passage is within the context of paying taxes and therefore refers to a nation's policies.  I am British, and unless I hold property in the US or Canada (which I don't), I am not answerable to the US or Canada when it comes to paying tax.  Similarly, as a Briton, I am not bound by American or Canadian law unless I happen to visit the country. 

This was an extra-judicial killing, carried out by forces of one country on a citizen of a second and within the borders of a third.

What message does that give to terrorists and, in all honesty, what makes it any different to the extra-judicial killings carried out by citizens on one or more nations, of citizens of several nations, within the borders of what (for many) was a third nation on 9/11?