The ruling says that INTENT cannot be questioned. The President can say whatever he/she wants after the assassination, and it cannot be questioned by courts. The Pres can say that the killing stopped an imminent terror attack. They can say the person was in the middle of committing a crime and had a (totally not planted) gun on them.
I get what you are saying, that extrajudicial execution is not a faculty given to the executive branch. In the US, the judicial system is supposed to have the power over adjudicating crimes. And US citizens have the right to trial by their peers. But the government has shown repeatedly in the past that when it comes to terror that they are more than happy to waive rights. See: Guantanamo, drone kills of US citizens, cops killing people who are only suspected of being a threat, etc.