Are the levers of government power pulled by those the governed have elected or those we don't see? Do elections change anything?

Can the president kill people whom he suspects might commit a crime? Aren't even those who would cause great harm entitled to due process? Isn't everyone entitled to a fair trial in front of a neutral judge and jury before any punishment can be administered?

Aren't all persons legally innocent until proven guilty beyond a reasonable doubt and to a moral certainty? Isn't this presumption of innocence the linchpin of American jurisprudence? At trial and before punishment, isn't it the government's obligation to prove every element of the crimes charged? Isn't there no such thing in American jurisprudence as a presumption of guilt?

Aren't punishments prescribed by law? Can the president make up a punishment and direct the military to administer it to folks he thinks are probably guilty of criminal behavior? Can federal officials perform unlawful acts with impunity just because they are ordered to do so by the president? Is "probably guilty" a sufficient legal standard for punishment?