Śrīla Viśvanātha Cakravartī Ṭhākura says in this connection,
- bhaktiṁ bhūtiṁ harir dattvā
- sva-vicchedānubhūtaye
- devyāḥ śāpena vṛtratvaṁ
- nītvā taṁ svāntike 'nayat
The purport is that the Supreme Personality of Godhead wanted to bring Citraketu to Vaikuṇṭhaloka as soon as possible. The Lord's plan was that Citraketu be cursed by Pārvatī to become Vṛtrāsura 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 Pārvatī to be embraced by Lord Śiva was natural in a relationship between husband and wife; this was nothing extraordinary for Citraketu to see. Nonetheless, Citraketu laughed loudly to see Lord Śiva 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.