Are you referring to someone making that claim publicly? For example a politician?

What did Trump post on Twitter? Did he threaten Macron?

Yes, It’s legal to kill them, too (with the specifications below), and Bush threatened Saddam (Declaration of War.)
Is that what you are asking?

“Saddam Hussein and his sons must leave Iraq within 48 hours. Their refusal to do so will result in military conflict commenced at a time of our choosing,”
https://www.theguardian.com/world/2003/mar/18/iraq.usa1

These commentators see a major distinction between intelligence
operations aimed at assassination and military operations aimed at
killing specific enemy combatants—including enemy leaders such as
Saddam Hussein and Colonel Qaddafi—in situations where the
United States is exercising its inherent right of self-defense.6 According to W. Hays Parks, the United States generally recognizes three
forms of self-defense: (1) “against an actual use of force, or hostile
act,” (2) “preemptive self-defense against an imminent use of force,”
and (3) “self-defense against a continuing threat.

https://www.rand.org/content/dam/rand/pubs/monograph_reports/MR1385/MR1385.ch2.pdf

It’s against the Geneva Convention. Not sure about specific US laws.

@KNOWITALL rand.org, really?

@gorillapaws It’s bias rating is great, link attached, and 53% have doctorates. It’s a global policy thinktank that leans more left of center than anything.

Are you just skeptical because it’s federally funded or what?

https://mediabiasfactcheck.com/rand-corporation/

I think the OP was asking whether an ordinary citizen could carry out an assassination of a foreign leader. That is an interesting question. It seems that it should be illegal, but I can’t say whether it is.

I did a Web search on killing of foreign leaders by presidents.

This article mentions an executive order by Gerald Ford. This would seem to be pretty weak. Executive orders by one president are frequently overruled by a succeeding president.

@KNOWITALL About 60% of it’s funding is from the Department of Homeland Securty, the Army, the Air Force and the State Department.

@gorillapaws Okay, then feel free to disregard it I guess…lol