As noted by nostramo [0], Obama already set the precedent of ordering hits on US citizens. The answer to Sotomayor's concern here seems pretty obvious: if we're concerned that the President can order hits on US citizens for invalid reasons, then we need to be very clear in the laws that ordering hits on US citizens without due process is not within the President's official authority.
I don’t think this is a helpful comparison. A citizen in the service of an enemy engaged in war against his country does not enjoy the protections of an arbitrary citizen. We can rightly argue whether that theory wholly fits the facts of al-Awlaki, but it’s a very, very long bridge from that case to Sotomayor’s hypo
Not really—once you've crossed that bridge it's a short hop to the government arguing that the political rival was a terrorist who needed to be killed.
Due process is about validating the government's claims before allowing it to kill someone.
[0] https://news.ycombinator.com/item?id=40849378