Well assassination of a political rival might be but what about the killing of the perceived head of a terrorist organization who attempted a coup to overturn a federal US election? Sounds like a national defense concern that the president could decide to handle. I think this is logically in line with the ruling but is completely absurd morally.

SCOTUS explicitly says you cannot question the motive of a presidents actions when making a determination for what is protected and not.

And how, pray tell, is that compatible with them also saying that neither Congress nor Courts can call into question the motivation of the President for doing so? All the president need do is say "they were a national security concern" and they are now absolutely immune from anyone so much as presenting evidence in a trial that says otherwise!

The ultimate Congressional remedy is impeachment, which this ruling doesn't contemplate (except to reject silly arguments presented by the defense). However, the limits on congress and the judiciary are not absolute:

> Congress cannot act on, and courts cannot examine, the President’s actions on subjects within his “conclusive and preclusive” constitutional authority. It follows that an Act of Congress — either a specific one targeted at the President or a generally applicable one — may not criminalize the President’s actions within his exclusive constitutional power. Neither may the courts adjudicate a criminal prosecution that examines such Presidential actions.

It's only the executives exclusive powers (which are more limited) that cannot be restrained.