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?
That logic doesn’t work because the official duties of the VP are both narrow and distinct from the President. Its not obvious that legal powers confer from the President to the Vice President in any way except under predefined circumstances. The VP would need to wait until those circumstances occurred, for example if the President was sedated for a medical procedure, and then do the official act while they have the actual powers of the Presidency behind them.