The onset of genocide of the natives of American continent and mass enslavement of Africans.
«Between 1500 and 1866, Europeans transported to the Americas nearly 12.5 million enslaved Africans, about 1.8 million of whom died on the Middle Passage, their bodies thrown into the Atlantic.»
Destruction of cultures, looting and spread of false religions.
Columbus returned home to Spain and came back to the Caribbean with 17 ships and 1,200 men. His men traveled from island to island, taking Indians as captives. In 1495, in a large slave raid, Columbus and his men rounded up 1,500 Arawak men, women, and children, and put them in pens. They selected what they considered the best natives and loaded them onto ships back to Spain. Two hundred died en route. After the survivors were sold as slaves in Spain, Columbus later wrote: «Let us in the name of the Holy Trinity go on sending all the slaves that can be sold.»
Columbus and his men had a particular reputation for cruelty. Bartolome de las Casas, a young priest who participated in the conquest of Cuba and wrote a history of the Indies, describes the treatment of the natives: “Endless testimonies… prove the mild and pacific temperament of the natives.… But our work was to exasperate, ravage, kill, mangle and destroy; small wonder, then, if they tried to kill one of us now and then… The admiral, it is true, was blind as those who came after him, and he was so anxious to please the King that he committed irreparable crimes against the Indians ...“ Las Casas describes how Spaniards rode on the backs of natives. How the Spaniards «thought nothing of knifing Indians by tens and twenties and of cutting slices off them to test the sharpness of their blades.» Las Casas adds «two of these so-called Christians met two Indian boys one day, each carrying a parrot; they took the parrots and for fun beheaded the boys.»
Facing extermination, the Arawaks organized and attempted to fight back against the Spaniards. But they were little match against the armor, muskets, swords and horses of the Europeans. The Spaniards hung or burned Indians that they took captive. By this point, the Arawaks began committing mass suicides. They fed cassava poison to their infants to save them from the Spanish. In two years, half of the 250,000 Indians on Haiti were dead, either through murder, mutilation or suicide. By 1550, there were 500 Indians. By 1650, the Arawaks had been wiped out from the island.
And much more…

Sources:

www.encyclopediavirginia.org/slave_ships_and_the_middle_passage

rapidcityjournal.com/lifestyles/people/top-5-atrocities-committed-by-christopher-columbus/collection_76ebb2b8-f63d-11e3-a137-001a4bcf887a.html#1