The president can’t kill anyone who opposes him. The president is subject to the laws just like anyone else. Breaking the law is not part of his official duty. Assassinating someone the president doesn’t like is against the law.
Assassinating an enemy of the United States is a different story. The president cannot claim a citizen of the United States with no criminal activity or record against the United States is an enemy. Furthermore, the military cannot use force on citizens of the United States. The FBI can, and the president doesn’t control the FBI the judicial branch does.