Basing absolute rule on the identity of a person's father is about as asinine a way of determining governance as imaginable.
Yet even with this isolation, allowing the common person a say in government has resulted in disaster. The common man operates almost entirely on the basis of emotion, and is easy for a demagogue to to manipulate. (Case in point: President Obama.) Rule on the basis of the "will of the people" is in reality rule by the most skillful demagogue.
As for atrocities: no king or emperor in history ever shed blood on the scale of a government by the people. A king's power is limited by the oath he swears to god; in a representative government, vox populi is vox dei, and as such is without limit. Hitler was, after all, an elected leader; Stalin is still loved by millions; the rule of both Mao Zedong and the Dear Leader were and are based largely upon the willing cooperation of millions of "jes' plain folks'. Even in America, we shed innocent blood on a scale that makes the worst excesses of the cruelest czar look like Sunday school. Get back to me when you find a king who allowed fifty million innocent babies to be legally murdered during his reign.