The Supreme Lord is the source of both evil and good as the purana stories in the Bhagavata Purana show. From time immemorial good and bad have coexisted in creation, and this is symbolically represented by the fact that Kashyapa is the father of both the devas, the sons of Aditi, and the asuras born of Diti. The Lord is the very embodiment of dharma and He sustains the entire creation. So, when evil dominates, He takes avatars to quell the asuras and restore the balance. Likewise in the Devi Mahatmya it is Goddess Shakti who intervenes to put an end to the atrocities perpetrated by the asuras such as Madhu and Kaitabha, Mahishasura, Sumbha and Nisumbha and so on, pointed out Sri R. Krishnamurthy Sastrigal in a discourse.
The purana stories also reinforce another important truth that each one’s essential nature is determined by his deeds and not by factors such as lineage, scholarship, wealth, etc. The asura clan is not totally bad for there are great bhaktas and good natured characters such as Prahlada and Bali. Also, there are instances where the Devas are not always good and at times err on the side of dharma. Once, when they became blind with conceit at their victory over the asuras, God taught them a lesson by making Bali overpower them. But, when the devas sought His help to save them from Bali’s dominance, Vishnu helped them and also rid Bali of his pride in his possessions.
Also, there are factors such as the cause and effect principle and the ebb and flow of Time in the lives of beings that give rise to favourable or unfavourable situations. This makes it appear as if God is partial to the devas rather than their opponents. But the truth is that God has no personal enmity towards anyone and His aim is to protect the good against the wicked forces.