When it came to torture (pleasantly known as enhanced interrogation), Dick Cheney insisted that it produced results, and therefore that it was justified. The movie, Zero Dark Thirty seems to imply a similar message. Now we are discussing targeted killings in the same way. They are justified on the basis that those killed are bad people and the country is protected from them as a result. But every constitutional violation could be rationalized in the same way. If the practical trumps principle than virtually any illegal government act can be justified.
We could find illegal guns, drugs, stolen property, and discover criminal activity from listening to conversations without authorization or warrant if we abandoned the 4th Amendment. We could save time and money if we were to forgo criminal trials. We could beat confessions out of suspects and stop advising them of their rights. More criminals would be found and more would be convicted and incarcerated.
But we are a country of morals and standards and rights. Our credo cannot be, "It's right because it works." Some argue that the evidence seems to indicate that torture doesn't work. Drone targeted attacks kill innocent people as well. Wiretaps listen in on millions of conversations between law-abiding citizens. But all of those discussions and debates have to do with some risk/benefit analysis, rather than whether the conduct is right or wrong irrespective of the good or harm it does.
But one might properly ask -- what about Osama bin Laden? It involves the same dilemma when discussing the death penalty. There are many times when a crime, a murder, or rape is so evil that even a death penalty opponent craves for the death of the perpetrator. But if one is opposed to the death penalty, deciding when to carve out exceptions is indeed that famous slippery slope. If killing someone, particularly an American citizen, without charge, a trial and a verdict is wrong, a few exceptions do not make it right. At a minimum, if the policy is to be pursued, there should be a judicial panel to review and pass upon the strength of the evidence and the need to bypass our most cherished rights.