"America ... goes not abroad in search of monsters to destroy." -- President John Quincy Adams (1767-1848)
The president's supporters have argued that the general's death was revenge for Americans and others killed by the general's troops and surrogates. Trump has argued, more importantly, that he ordered the general's death because of what evil the general might order his own troops and surrogates to do in the future.
Can the president legally kill a person not engaged in an act of violence because of what the person might do in the future? In a word: No.
The president has taken an oath to uphold the Constitution. The Constitution provides only two means for the federal government to kill a human being. The first is pursuant to a declaration of war, which only Congress can do. That permits the president to use the military to kill the troops of the government of the country against which war has been declared. Congress has not declared war on Iran.
The second way that the Constitution permits federal government killings is pursuant to due process. That means that the person to be killed is lawfully in custody, has been properly charged, lawfully tried and fairly convicted of a capital crime, and the conviction has been upheld on appeal.
Roaming the world looking for monsters to slay not only violates
long-standing principles of American domestic and international law, but
also it violates basic Judeo-Christian moral principles, which teach
that the end does not justify the means and might does not make right.
Think
about it. If the American president can kill an Iranian government
official in Iraq because of fear of what he might do -- without a
declaration of war or any legal process -- can the Chinese president
kill a Mexican government official visiting in Texas or an American
intelligence agent encouraging revolution in Venezuela for fear of what
they might do?