But that is the question. Does it change that? In my previous post, I asked (or more correctly, a Justice asked, I just fleshed it out) about bribery. Is is OK for a president to accept a bribe to perform an official act? It's the exact same question as asking if a President can order the killing of a US citizen on US soil.

It's not the same question.

In the bribery case, there is a legal official act involved. For example, if the President accepts a bribe to grant a pardon, there is no question that the act being performed - granting a pardon - is within the power of the President. Granting the pardon was absolutely within his power, and the question is whether exercising that power for an illegal/illicit purpose can be criminalized.

But in the assassination case, there isn't a legal official act. The President has no power to assassinate a U.S. citizen on U.S. soil. An extrajudicial killing isn't merely outside of the President's lawful authority; it's forbidden to the President (or anyone in the federal government) to do that under the 14th Amendment. There is no exercise of lawful executive power that the President can point to as the basis for immunity.