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!"