The President is the commander-in-chief of the armed forces. Why would he have to stop at only his domestic rival when he could instead simply eliminate anyone

The President is the commander-in-chief of the armed forces. Why would he have to stop at only his domestic rival when he could instead simply eliminate anyone who appears to be in the way, and whether or not that is the right decision is deferred to congress and the courts who now consist of people who are still alive and have fallen in line?