How you and me came to this world... over 12 billion years, from mere dust , to the simplest form of cells to plant life to aquatic mammals to apes to you & me. Which means there is constant 'upgradation' of species .Over millions of years. each of the species , learn, adapt, grow in intelligence to graduate into the next higher form of life. This would mean in humans, there would be large gradation - those who have just entered human species for the first time, from being an ape earlier compared to those who have been humans for over thousands of cycles of birth & death .Some of these souls are likely to continuously  develop their consciousness ( e.g Mother Theresa, Swami Vivekanand etc) 

And hence the humanity would always comprise of humans with large proprtion of animal instinct , who would indulge in violence, hatred etc.