In Mahabharat, Shakuni is considered as one of most negative person who was ruining the minds of Kauravas. But, we never got the exact reason behind his such a poisonous thoughts. But, we have the answer and the story goes like this.
Bheeshma once lead a war against the kingdom of Gandhara and conquered it. He killed the king and captive all the male members of his line, together with Shakuni the aristocrat and his 100 brothers.
Since all of them were fed with only one morsel of rice on a daily basis, the family determined that a minimum of one in every of them may survive and gave all the rice to Shakuni, the foremost cunning and youngest of all of them.
Shakuni lived on and came to Hastinapur and brainwashed Duryodhan from a spinnable age, instigating him to cause the Kurukshetra war by repeatedly pointing him that the Kuru tribe would be destroyed by Pandavas.