Obama started killing Americans with Predator drones more than 15 years ago, and nobody has dared to prosecute him. So it is already stablished that POTUS can kill Americans at will. The American Civil Liberties Union sued the government because of the targeted killings of three American citizens, some of them who hadn't even been accused of commitingcommitting any crimes. The Obama administration claimed its right to kill whoever they wanted to, without restrictions or supervision.
The most the ACLU got from the courts was the government producing a document detailing the alibis that could be used to justify killing an American citizen at will. At present, it would be enough to label the target as "terrorist", and then you're safe to go and kill itthe target - there are no prerequesitesprerequisites to being considered a terrorist.