Congress didn't as much grant this authority as legitimized abuses already in progress. The USA has had assassination programs since the 1970s at least, probably since the 1950s. Both the CIA and the FBI undertook political operations within the USA as far back as the 1960s. They were theoretically shut down in the early 1980s, but there were still rumblings and the rumblings kept getting louder. (An introduction, here.) So it was all ready to go on 9/11.
It is worth remembering whose appointment, and to what office, Paul was filibustering. John Brennan, now confirmed as CIA director, was one of the architects of the expansion of the national security state under Bush II, and now Obama. In particular he is the man making drone assassination decisions. And he has a history. Marcy Wheeler, interviewed on Democracy Now in January:
[...] he was George Tenet’s chief of staff and then went on to be kind of the precursor to what’s now called the National Counterterrorism Center. And in that role, he touched the early moments of the torture program and, I think more importantly, was involved in the targeting for Cheney’s illegal wireless—warrantless wiretapping program, when it was working without any legal sanction at all. So, as you said at the top of this story, Brennan was considered unacceptable four years ago to lead the CIA. Since then, we’ve learned about his role in the illegal wiretapping program. We’ve learned about his role in drone strikes. And yet, now he’s supposed to be an acceptable candidate? I don’t understand how—I mean, I guess that’s a testament to what kind of hard-nosed person Barack Obama has become and the degree to which his policies really are just a continuation of the Bush-Cheney policies.When Attorney General Holder uses the word "American" in talking about Fifth Amendment rights, rather than "person," that is a matter of concern. Surveillance drones are already being used on the Mexican border and in Colombia. The idea of deploying armed drones in Latin America, based outside of the USA, and therefore under no restrictions of US domestic law, has surely already been floated in Washington; for all I know it has already begun. The potential for atrocities, the sorts of things which go down in history as things to wonder at in horror, is enormous. We need foreign policy and, eventually, international law to prevent this, and I can only hope that sanity will break out, and turn the USA away from further abuses of armed drones.
I worry that no leader now will start the international debate. For the moment, the administration, through Holder, has renounced the use of armed drones against US citizens in the USA, and I think this will be made law (but when has the CIA ever been respectful of law?) This is not enough. What has been done with drones in Gaza and Pakistan ought to be banned by international treaty, and with Brennan as Director of the CIA, the USA is not likely to take the lead on this.