Visnu is always in favor of the demigods because demigods, they are also devotees. The difference between sura and asura... Asura means envious of the demigods. That is called asura. Sura-dvisa. The another name of the asura, "those who are envious of devotees." That is the only business.
Lecture on Srimad-Bhagavatam 1.3.19, Los Angeles, September 24, 1972