PURPORT

Although Kamsa was a sinful demon, he believed that Vasudeva would never deviate from his word. The character of a pure devotee like Vasudeva is such that even so great a demon as Kamsa firmly believed in his words and was satisfied. Yasyasti bhaktir bhagavaty akincana sarvair gunais tatra samasate surah (Bhag. 5.18.12). All good attributes are present in a devotee, so much so that even Kamsa believed in Vasudeva's words without a doubt.