I just read “China's Bloody Century” by the late RJ Rummel. The communist government killed almost 40 million of its own people. Rummel wonders why:

"How do we explain such killing? It is popular in the literature on genocide to underline causes like dehumanization, segregation, group polarization, racism, a subject majority, hegemonic drive, threat to the power structure, "outsider" minorities, ethnic/racial/religious stratification, primitiveness, retribution, development, scapegoatism, ideology, cultural clash, war/revolution, among others."

Rummel concludes:

"Power kills. And absolute power kills en masse. Consider--the Soviet Union, 61,911,000 people murdered; the Chinese communists, 38,000,000; the Nazis, 17,000,000; the Chinese Nationalists, 10,214,000; the Japanese militarists in World War II, 5,890,000; the Khmer Rouge of Cambodia, 2,000,000; West Pakistan over East Pakistan, 2,000,000; Turkey during and after World War I, 1,500,000; and Yugoslavia after World War II, 1,105,000. But this equation of power is not limited to democide. Power's human destructiveness is perfectly general. Power also breeds violence and war and all their associated killing. War, revolution, and democide are as natural to power as the lust for power is to our species".

How can we prevent any one individual or group from acquiring absolute power? In a word: competition. That is, counter-forces enabled by institutions, laws, and culture that allow competing centers of power and influence to flourish, e.g., separation of powers, protections against majority rule, multiparty politics, states’ rights, free speech, competitive markets, respect for the scientific method, etc. But there is no guarantee against absolute power, because control/power feels so good yet ceases to be satisfying when it becomes too easy. The lust for power wants to extend its reach, to stretch what it thought was possible, to take on new challenges, to feel that rush again. 

Opportunity quickens desire and power creates opportunities. And so the dream of absolute power continues to beckon.