The purport is that the Supreme Personality of Godhead wanted to bring Citraketu to Vaikunthaloka as soon as possible. The Lords plan was that Citraketu be cursed by Parvati to become Vrtrasura so that in his next life he could quickly return home, back to Godhead. There have been many instances in which a devotee acting as a demon has been brought to the kingdom of God by the mercy of the Lord. For Parvati to be embraced by Lord Siva was natural in a relationship between husband and wife; this was nothing extraordinary for Citraketu to see. Nonetheless, Citraketu laughed loudly to see Lord Siva in that situation, even though he should not have done so. Thus he was eventually cursed, and this curse was the cause of his returning home, back to Godhead.