The President's power as Commander in Chief is enumerated, so having Seal Team 6 whack DeSantis would be absolutely immune to prosecution.
I wouldn't say that the history and tradition of the office is to have Delta Force manufacture artistic pottery cups for children to drink milk from while at school. That action in that time and place would not seem to fall within the role of Commander in Chief (regardless of his intent / subjective belief that the use of fine porcelain may provide some tactical advantage).
That was the whole point of the decision - the official in question gets to determine what is/isn't an official action.
This does not seem to fit either the examples set in the majority opinion or a reasonable understanding of what the President's enumerated power as commander in chief means. In the absence of a Congressional declaration of war against Ron DeSantis (which I am all for) the President's core duty would seem to cover killing DeSantis only in the event of a clear and present danger to the nation from DeSantis. Otherwise this is rebuttable, and will be pretty easily rebutted, immunity.
Yes, we are relying on the courts to carve that kind of fine distinction, and yes, this decision makes that a bit harder. But hardly impossible, and we were already relying on the courts for that. Subjecting this decision to the most ridiculous interpretation is politically useful right now but that's about all it's good for.
This court would rule that people upset about the president assassinating all their rivals should just get elected president and change the policy.
After all, they already did say that women that want health care and the right to prevent (according to the court) overwhelmingly male politicians from messing with their lady bits should just run for Supreme Court and overturn their recent rulings.