Now visualize a psychopath inheriting the throne. (then again, democracy DID produce Clinton...)
Why visualize it? Why not just cite an historical example from some time in the last two hundred years?
Check out Professor Rummel's work on twentieth century democide and compare how many monarchies are listed, vs. "popular" and revolutionary governments. And it's not as if monarchs did not have the means of mass murder at their disposal. Attila the Hun did a pretty good job using ingredients commonly found in any kitchen, and even prior to the invention of barbed wire.
The most recent "notorious" royal (alleged) lunatic was Ludwig II of Bavaria, who nevertheless left us with Neuschwannstein castle and the Baireuth Festival. Not a death camp in sight, nor any starving Bavarians working in salt mines.
There is a much greater chance that people will elect a psychopath (twice, in the case of Clinton) than that he would be allowed to rule in an hereditary monarchy. The reasons are family pride, and the intention to hand on an intact patrimony to the next in line. Royal families have a way of compensating for mental deficients, even when they are in the direct line of succession. Sometimes this is done formally through a Regency, sometimes informally.