PURPORT

Although Vrtrasura repeatedly encouraged Indra to kill him with the thunderbolt, King Indra was morose at having to kill such a great devotee and was hesitant to throw it. Vrtrasura, disappointed that King Indra was reluctant despite his encouragement, took the initiative very forcefully by throwing his trident at Indra. Vrtrasura was not at all interested in victory; he was interested in being killed so that he could immediately return home, back to Godhead. As confirmed in Bhagavad-gita (4.9), tyaktva deham punar janma naiti: after giving up his body, a devotee immediately returns to Lord Krsna and never returns to accept another body. This was Vrtrasura's interest.