I don’t think assassinations of political rivals would be covered under the president’s constitutional duties.
But national security is. All they would need is a flimsy justification that the person was stealing state secrets (like Trump) or organizing a terrorist attack, which could include any contact with an armed or paramilitary group that’s planning a protest. They could use state influence to coerce that group to take action, and the records of that planning process would be inadmissible per this ruling. It’s not hard to come up with superficial reasons that do align with Constitutional obligations.
Edit to add: Hell, just look at the McCarthy era, or the Iraq war. It’s not hard at all for a sufficiently shameless group of politicians to gin up a moral panic about national security. They don’t even need evidence, they just need motive. We’re real fucking close to the government being able to legally assassinate purported communists for subversion.
Just because national security is the domain of the Executive doesn’t mean they can use lethal force on anyone they wish in any scenario they wish in lieu of effecting arrests for alleged crimes.
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.
I mean, they have to sign some paperwork to make it an official act, but otherwise what’s the difference? They don’t have to arrest anyone according to this ruling, if I’m reading this correctly. Sure, us normal citizens probably do, but according to the court, presidents don’t have to follow the law if it’s an official act. That’s kind of the basis of the dissent. It separates the rules we follow and our leaders have to follow.