Lord Vishnu has been destroying adharma since his first avatar and he keeps on destroying adharma by promoting dharma. But how was adharma born in humans? Aren't humans the creation of god?
In Hinduism, Lord Brahma is the creator or the Srishtikarta. So, the whole creation emerges from him.
All pairs and opposites like- day-night, Papa-Punya, Adharma-Dharma, are created from him only.
From Lord Brahma's back side, inauspicious entities, like Alakshmi and Adharma, are born.
From Brahma Vaivarta Purana:
.... the Chief of Matrikas. Savitri also produced from her breast-milk three Kalpas viz.Brahma, Padma and Vaaraaha; four Pralayas viz. Nitya, Naimitthika, Dwiparaartha and Prakrita, Kaala, Mrityu Kanya and Vyadhiganas. Later on Brahma continued his Srishti from his Prishta (rear side) the two entities -Adharma and Alakshmi; from his nabhi (navel) Vishwakarma the Shilpa Guru, from his Balaparaakrama or prowess the Eight Vaus; Manasa Putras (mind-born sons) Sanaka, Sanandana, Sanatan....