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?
Also, impeachment. One would like to think that even today, a president-through-killing-the-president-while-being-vice-president would be impeached and removed by congress, but who knows.
Yes, but Republicans do what they want with no consequences, so check and mate.