SB 10.4.42
sa hi sarva-suradhyakso
hy asura-dvid guha-sayah
tan-mula devatah sarvah
sesvarah sa-catur-mukhah
ayam vai tad-vadhopayo
yad rsinam vihimsanam
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Lord Visnu, the Supersoul within the core of everyone’s heart, is the ultimate enemy of the asuras and is therefore known as asura-dvit. He is the leader of all the demigods because all the demigods, including Lord Siva and Lord Brahma, exist under His protection. The great saintly persons, sages and Vaisnavas also depend upon Him. To persecute the Vaisnavas, therefore, is the only way to kill Visnu.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The demigods and the Vaisnavas especially are part and parcel of the Supreme Lord, Visnu, because they are always obedient to His orders (om tad visnoh paramam padam sada pasyanti surayah). The demoniac followers of Kamsa thought that if the Vaisnavas, saintly persons and sages were persecuted, the original body of Visnu would naturally be destroyed. Thus they decided to suppress Vaisnavism. The asuras perpetually struggle to persecute the Vaisnavas because they do not want Vaisnavism to spread. Vaisnavas preach only devotional service, not encouraging karmis, jñanis and yogis, because if one must liberate oneself from material, conditional life, one must ultimately become a Vaisnava. Our Krsna consciousness movement is directed with this understanding, and therefore the asuras always try to suppress it.