This verse is from the Bhakti-rasāmṛta-sindhu (1.2.278) of Śrīla Rūpa Gosvāmī, who further discusses this same topic in his Laghu-bhāgavatāmṛta (Pūrva 5.41). There he refers to the Viṣṇu Purāṇa (4.15.1), where Maitreya Muni asked Parāśara, in regard to Jaya and Vijaya, how it was that Hiraṇyakaśipu next became Rāvaṇa and enjoyed more material happiness than the demigods but did not attain salvation, although when he became Śiśupāla, quarreled with Kṛṣṇa and was killed, he attained salvation and merged into the body of Lord Kṛṣṇa. Parāśara replied that Hiraṇyakaśipu failed to recognize Lord Nṛsiṁhadeva as Lord Viṣṇu. He thought that Nṛsiṁhadeva was some living entity who had acquired such opulence by various pious activities. Being overcome by the mode of passion, he considered Lord Nṛsiṁhadeva an ordinary living entity, not understanding His form. Nevertheless, because Hiraṇyakaśipu was killed by the hands of Lord Nṛsiṁhadeva, in his next life he became Rāvaṇa and had proprietorship of unlimited opulence. As Rāvaṇa, with unlimited material enjoyment, he could not accept Lord Rāma as the Personality of Godhead. Therefore even though he was killed by Rāma, he did not attain sāyujya, or oneness with the body of the Lord. In his Rāvaṇa body he was too much attracted to Rāma’s wife, Jānakī, and because of that attraction he was able to see Lord Rāma. But instead of accepting Lord Rāma as an incarnation of Viṣṇu, Rāvaṇa thought Him an ordinary living being. When killed by the hands of Rāma, therefore, he got the privilege of taking birth as Śiśupāla, who had such immense opulence that he could think himself a competitor to Kṛṣṇa. Although Śiśupāla was always envious of Kṛṣṇa, he frequently uttered the name of Kṛṣṇa and always thought of the beautiful features of Kṛṣṇa. Thus by constantly thinking and chanting of Kṛṣṇa, even unfavorably, he was cleansed of the contamination of his sinful activities. When Śiśupāla was killed by the Sudarśana cakra of Kṛṣṇa as an enemy, his constant remembrance of Kṛṣṇa dissolved the reactions of his vices, and he attained salvation by becoming one with the body of the Lord.
From this incident one can understand that even a person who thinks of Kṛṣṇa as an enemy and is killed by Him may be liberated by becoming one with the body of Kṛṣṇa. What then must be the destination of devotees who always think favorably of Kṛṣṇa as their master or friend? These devotees must attain a situation better than Brahmaloka, the impersonal bodily effulgence of Kṛṣṇa. Devotees cannot be situated in the impersonal Brahman effulgence, into which impersonalists desire to merge. The devotees are placed in Vaikuṇṭhaloka or Kṛṣṇaloka.