"[W]hat is government itself, but the greatest of all reflections on human nature? If men were angels, no government would be necessary. If angels were to govern men, neither external nor internal controls on government would be necessary. In framing a government which is to be administered by men over men, the great difficulty lies in this: you must first enable the government to control the governed; and in the next place oblige it to control itself" (Federalist 51, https://www.ourdocuments.gov/doc.php?flash=false&doc=10&page... ).
The Constitution was designed with the assumption that the people in any branch of the government wouldn't necessarily be "the right people," and the other branches would need to serve as watchdogs (even though the other branches wouldn't necessarily be run by the right people either; in the worst case, ruthless politicians in Congress are expected to keep tabs on a ruthless president, and a ruthless president is expected to keep tabs on a ruthless Congress).
There are mistakes in the original Constitution (e.g., the elections of 1796 and 1800), but if today's Constitution relies on the people electing angelic politicians, then we've done something seriously wrong.
Having Congress and the president keep each other in check appears to work in practice as long as they are controlled by different parties. Of course, they are currently controlled by the same party, so there is good reason to worry. The only real solution in that case is to not give power to the president if it bothers you that the person in the Oval Office might not always be someone you like. If you don't like the fact that President Trump can order the assassination of an American citizen, the time to speak up is when President Obama first claims that authority ( http://www.salon.com/2010/02/04/assassinations/ ).