Has the recent big Supreme Court ruling changed what a sitting president can do or what can be done to a sitting president?

I would think poisoning civil servants would NOT be an official act. The caution people are discussing is along the lines of the president doing what he is entitled to do - order the army to kill someone, or order US Marshals to arrest someone and throw them in jai - the president is entitled to issue those ordersl. Sell pardons for bribes? The president is entitled to issue pardons.

(My favourite scenario… the president orders John Doe arrested on suspicion of being a terrorist, throws him into the worst Max Security. Sure, the guy’s lawyers will have him sprung in a few days - but then he simply gets arrested again. And again. and again… The court cannot prosecute for false imprisonment, cannot question the grounds for arrest decalred by POTUS as head fo the DoJ, etc. Cannot hold POTUS in contempt for ignoring court orders to stop harrassing JD. His property can be seized and bank accounts frozen, to sit idle while the litigation continues. How long in jail or items held of course relies on the POTUS lawyers being experts at delay, delay, delay… what are the odds? )

If it’s an official act, the president can do it. The act cannot be entered as evidence is court, making it very difficult to prosecute him. If he pardons the Seal Team 6, then they don’t get prosecuted either. Presumably as federal officials, any state case against ST6 for murder would be moved to federal court. Not sure how that plays out with pardons, etc. Similarly, not clear if the “cannot be entered to evidence” rule applies to civil lawsuits, when the assasinated person’s family sues. Can they sue the (ex)president? Can they sue the army/navy/marines whodunnit?

Probably military court, but as we have seen the president is perfectly willing and able to pardon war criminals found guilty by a court martial, so your scenario is entirely possible.

Among other possibilities is Putin offering Trump a billion dollars to be placed in his personal bank account if he sends a series of F-16 strikes against Ukrainian positions. Completely non-prosecutable.

Yes, it would go without saying the president would need to also pardon his henchmen for any actions he was immune to. But remember too a pardon removes the person’s right to plead the fifth in regard to his actions.

My point was the president cannot pardon state crimes. Murder in a state is a state crime. (Murder per se is not a federal crime, IIRC, unless the person murdered is a federal employee?) IIRC, IANAL, etc. there’s a procedure where state charges are moved to federal court if the charges are against federal officials in the course of their duties (which Meadows tried in Georgia, but they said that phone call was not official business). At the time they mentioned it was an odd procedure where a state prosecutor prosecuted the state crime statute but with a federal judge. I’m not clear if that’s a case where the charge can be pardoned by a president.

Also, if there are competing federal and state crimes, does the fed always take precedence? Does a pardon cause the state crime to be prosecutable instead?

So, don’t assassinate anyone who is not currently a federal official and don’t do it except in DC.

There’s an additional wrinkle that’s being mentioned now over current cases - the evidence that the president ordered an official act, and his motives, are not admissible evidence in any prosecution against him. However, the determination of what exactly the boundaries of a specific act’s “officialness” is something the court (and ultimately SCOTUS) needs to decide in each case; and to do so, the trial judge may hold a hearing, essentially a mini-trial, to determine what exactly transpired and whether it should essentially be “official”. At which point, numerous parties may or may not be compellled to testify as to the actions taken and the circumstances. So no criminal prosecution, but plenty of daylight.