Why six sons of Devaki & Vasudeva before the birth of Balram & Krishna were killed by kamsa?

Formerly an asura named Kālanemi had six sons, named Haṁsa, Suvikrama, Krātha, Damana, Ripurmardana and Krodhahantā. They were known as the ṣaḍ-garbhas, or six garbhas, and they were all equally powerful and expert in military affairs. These ṣaḍ-garbhas gave up the association of Hiraṇyakaśipu, their grandfather, and underwent great austerities to satisfy Lord Brahmā, who, upon being satisfied, agreed to give them whatever benediction they might desire. When asked by Lord Brahmā to state what they wanted, the ṣāḍ-garbhas replied, “Dear Lord Brahmā, if you want to give us a benediction, give us the blessing that we will not be killed by any demigod, mahoraga, Yakṣa, Gandharva-pati, Siddha, Cāraṇa or human being, nor by great sages who are perfect in their penances and austerities.” Brahmā understood their purpose and fulfilled their desire. But when Hiraṇyakaśipu came to know of these events, he was very angry at his grandsons. “You have given up my association and have gone to worship Lord Brahmā,” he said, “and therefore I no longer have any affection for you. You have tried to save yourselves from the hands of the demigods, but I curse you in this way: Your father will take birth as Kaṁsa and kill all of you because you will take birth as sons of Devakī.” Because of this curse, the grandsons of Hiraṇyakaśipu had to take birth from the womb of Devakī and be killed by Kaṁsa, although he was previously their father.

Source:A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Tenth Canto, Chapter 01 – Additional Notes