The killing of the six sons of Vasudeva is significant. They represent śabda, sparśa, rūpa, rasa, and gandha — the five sense objects sound, touch, form, taste, and smell, plus the wicked mind. Unless these six are killed, you cannot attain Krishna. Your heart will not be in śuddha-sattva, so how can Krishna appear? So these six sons were killed prior to the appearance of Lord Balarama. These six were previously six sons of Marichi Muni. They were cursed, so they came in the womb of Devaki and were killed by Kamsa. And after Kamsa killed them they stayed with Bali Maharaj in Sutala.