Jonathan Tobin at Commentary explains that the targeted assassination of Iranian nuclear scientists is not "terrorism":
But you need a particular form of moral myopia not to see that heading off a potential second Holocaust in the form of an Iranian nuclear attack on Israel or the nuclear blackmail of the rest of the Middle East is not a form of terrorism.
From U.S. Code Title 22, Ch.38, Para. 2656f(d) (d) Definitions
As used in this section—
(1) the term “international terrorism” means terrorism involving citizens or the territory of more than 1 country;
(2) the term “terrorism” means premeditated, politically motivated violence perpetrated against noncombatant targets by subnational groups or clandestine agents;
Now, I am not a lawyer and it may be that other caveats and exceptions cover state-sponsored acts when the state is the US or an ally. But the part of the code I am looking at does not seem to include a "good guys" exception.
Now, maybe it ought to - I can't say I am too worked up about these killings, whether they are being done by the Israelis, the US, both, or with the aid of even more parties.
That said, I believe we are stuck on the terrorist-freedom fighter puzzle that has prevented the UN from agreeing to a definition of "terrorism".