It was created by the sadistic psychopath, King Leopold II of Belgium, who made it his personal property and enslaved nearly the entire population to harvest ivory and wild rubber.  Up to half the population died.

After an international outcry, Leopold swindled the Belgian government into buying the territory from him for a lavish sum.  Outright genocide dwindled out, but forced labor and terror remained a hallmark of Belgian rule until independence.

Did I say "independence"?  In name only.  Belgium and the US arranged for the brutal assassination of democratically elected Patrice Lumumba.  They installed Joseph Mobutu, a kleptocrat who robbed the country blind and impoverished his people, but was a reliable Cold War ally.

Mobutu was overthrown in 1997 by Rwanda and Uganda, eager to get their hands on Congo's vast mineral wealth.  Brutal massacres and forced labor followed.  Rwanda and Uganda eventually began fighting each other over mining rights.  Several other neighbors joined in, anxious for their share of the loot.  Ethnic disputes spiralled out of control and the country was reduced to barbarism.

Four million people died.  

Western companies blithely bought coltan, diamonds, and other minerals from the various warlords, not caring if people had died mining them.

Now there is peace, of a sort, but many militias remain armed.  Warlords are now cabinet ministers and presidential candidates.  The election is a great step forward, but a truly stable democracy is a very long way off.