Asuras and Devas both trace their lineage back to Rishi Kashyapa - Asuras being the sons of Kashyapa and Diti while Devas being the sons of Kashyapa and Aditi. So why did the Asuras choose the path of Adharma (with exceptions like Mahabali) and Devas choose Dharma and gained administratorship over the cosmos as well as Amrutha?
The nature of asuras is such because they were conceived in the womb of Diti at an improper time. Desiring to have children and overcome by lust Diti approached sage Kashyapa at the evening time when ghosts, demons, etc. were wandering lead by Shiva, the lord of ghosts. So Kashyapa told Diti to wait for sometime because that time was inauspicious:
This particular time is most inauspicious because at this time the horrible-looking ghosts and constant companions of the lord of the ghosts are visible. [SB - 3.14.23]
But as struck by lust, Diti approached sage Kashyapa neverthless. Knowing the course of fate Sage Kashyapa fulfilled the desire of Diti but after it he told her the consequence of it:
The learned Kasyapa said: Because of your mind's being polluted, because of defilement of the particular time, because of your negligence of my directions, and because of your being apathetic to the demigods, everything was inauspicious. [SB - 3.14.38]
O haughty one, you will have two contemptuous sons born of your condemned womb. Unlucky woman, they will cause constant lamentation to all the three worlds! [SB - 3.14.39]
So two demons Hiranyaksa and Hiranyakasipu were born to her from which the race of the asuras grew. But as Diti regretted and because she had faith in the Lord, Sage Kashyapa told that her grandson would be a great devotee of the Lord:
The learned Kasyapa said: Because of your lamentation, penitence and proper deliberation, and also because of your unflinching faith in the Supreme Personality of Godhead and your adoration for Lord Siva and me, one of the sons [Prahlada] of your son [Hiranyakasipu] will be an approved devotee of the Lord, and his fame will be broadcast equally with that of the Personality of Godhead. [SB - 3.14.44,45]
So Prahlada, although was born in the race of the asuras was a great devotee and some of his attributes were also present in king Bali, the grandson of Prahlada.