So the supreme court already ruled the president cannot be held accountable for anyone they kill.
The vice president becomes the president instantly if the president dies.
What is preventing any vice president from waiting until day 1 of their parties presidency, and then murdering the president? And then instantly pardoning themself?
I’m sure analogous situations have come up many times in history, such as in the Roman empire. The Borgia clan in the Italian Renaissance got a possibly unjust reputation for stuff like that. Machiavelli’s “The Prince” was supposedly inspired by Cesare Borgia.
Part of the answer has been to be careful who you pick as VP. Trump picked Pence partly because Pence (at the time) seemed to have no presidential ambitions. It worked reasonably well, since Pence could have (politically) put the shiv into Trump during either of Trump’s two impeachments. If Trump becomes president again, he’ll likely get impeached at least a couple more times, so we’ll see if Vance is similarly loyal.