It is said in the Kalki Purana --> "According to Kalki Purana, Kali demon will rule the earth and bring sin and misery to the people. When the first stage of the Kali Yuga begins, the varna system will deteriorate. Soon after this, the second, third, and fourth stages of the Kali Yuga begin. During the second and third portions, men will forget the name of God and no longer offer Yagna to the Devas."
But why would Kali demon rule earch and bring sin and misery? For what reason?
It's part of the demon Kali's fundamental nature to be evil, considering his lineage, which described in this chapter of the Srimad Bhagavatam:
Another son of Lord Brahmā was Irreligion, whose wife’s name was Falsity. From their combination were born two demons named Dambha, or Bluffing, and Māyā, or Cheating. These two demons were taken by a demon named Nirṛti, who had no children. O great soul, from Dambha and Māyā were born Greed and Nikṛti, or Cunning. From their combination came children named Krodha [Anger] and Hiṁsā [Envy], and from their combination were born Kali and his sister Durukti (Harsh Speech). O greatest of all good men, by the combination of Kali and Harsh Speech were born children named Mṛtyu (Death) and Bhīti (Fear). From the combination of Mṛtyu and Bhīti came children named Yātanā (Excessive Pain) and Niraya (Hell).
And Kali certainly lived up to his pedigree; this chapter of the Srimad Bhagavatam describes how Kali was trying to destroy the last leg of the bull of Dharma. (See my answer here for a description of the four legs.)
By the way, it should be noted that the sinful nature of Kali aside, the Kali Yuga is not all negative; see my answer here for the benefits of living in the Kali Yuga, such it made Vyasa shout out "Excellent, excellent, is the Kali age!"
Whatever answer you all gave are analogical. Here is right answer: In his past life king kali was a Gandharba. Gandharba means the children of demigods(demigods are in charge of universal administration residing in higher planets. So kali was a very powerful gandharba. He loved another damsel who was filled with all devine qualities. But instead of kali she preferred to marry a human, who was ruling over earth. (Kali had some other name that time). Due to this kali became envious and harrased them badly. The king lost everything and begged even for a piece of cloth. Finally the lady curshed kali to become degraded. So kali now came as a ruller in his age and has a aim to take revenge on humans. He have power to control orhers mind and he makes every human fight in the name of religion and color and kill each other. Do all wrong and go to hell. Kali has only two weaknesses. One: He can not harm a person who regularly reads Bhagavatam. 2nd weakness is still unknown and hidden in Bhagavatam itself.
