Supreme Court rules ex-presidents have immunity for official acts

That core concept of immunity is pretty solid and essential, it is the details that are problematic, in particular the fact that the courts have interpreted "official" acts so broadly in the past in cases of qualified immunity makes one worried they will do the same here.

Starting on page 44 of the opinion, Thomas makes some very good points.

    I write separately to highlight another way in which this prosecution may violate our constitutional structure. In this case, the Attorney General purported to appoint a private citizen as Special Counsel to prosecute a former President on behalf of the United States. But, I am not sure that any office for the Special Counsel has been “established by Law,” as the Constitution requires. Art. II, §2, cl. 2. By requiring that Congress create federal offices “by Law,” the Constitution imposes an important check against the President—he cannot create offices at his pleasure. If there is no law establishing the office that the Special Counsel occupies, then he cannot proceed with this prosecution. A private citizen cannot criminally prosecute anyone, let alone a former President. 
    
    No former President has faced criminal prosecution for his acts while in office in the more than 200 years since the founding of our country. And, that is so despite numerous past Presidents taking actions that many would argue constitute crimes. If this unprecedented prosecution is to proceed, it must be conducted by someone duly authorized to do so by the American people. The lower courts should thus answer these essential questions concerning the Special Counsel’s appointment before proceeding.
    
    ...
    
    Even if the Special Counsel has a valid office, questions remain as to whether the Attorney General filled that office in compliance with the Appointments Clause. For example, it must be determined whether the Special Counsel is a principal or inferior officer. If the former, his appointment is invalid because the Special Counsel was not nominated by the President and confirmed by the Senate, as principal officers must be. Art. II, §2, cl. 2. Even if he is an inferior officer, the Attorney General could appoint him without Presidential nomination and senatorial confirmation only if “Congress . . . by law vest[ed] the Appointment” in the Attorney General as a “Hea[d] of Department.” Ibid. So, the Special Counsel’s appointment is invalid unless a statute created the Special Counsel’s office and gave the Attorney General the power to fill it “by Law.” 
    
    Whether the Special Counsel’s office was “established by Law” is not a trifling technicality. If Congress has not reached a consensus that a particular office should exist, the Executive lacks the power to unilaterally create and then fill that office. Given that the Special Counsel purports to wield the Executive Branch’s power to prosecute, the consequences are weighty. Our Constitution’s separation of powers, including its separation of the powers to create and filled offices, is “the absolutely central guarantee of a just Government” and the liberty that it secures for us all. Morrison, 487 U. S., at 697 (Scalia, J., dissenting). There is no prosecution that can justify imperiling it.

> The immunity the Court has recognized therefore extends to the “outer perimeter” of the President’s official responsibilities, covering actions so long as they are “not manifestly or palpably beyond [his] authority.”

I’m hoping that the assassination of a political rival would be “palpably beyond” the authority of the President.