So Arjuna belonged to the group of men who was meant for giving protection to the society. So when he was declining to fight, Arjuna, when he was declining to fight, at that time Kṛṣṇa gave him advice that "It is your duty to fight." So generally killing is not good at all, but when there is enemy, aggressor, then to kill the aggressor is not sin. So the other party in the Battlefield of Kurukṣetra, they became aggressor to the Arjuna's party. Now, this is the set up of the Bhagavad-gītā. The real purpose is to instruct Arjuna about spiritual understanding.
So spiritual understanding means first of all to know what is spirit. If you do not know what is spirit, then where is spiritual understanding? People are too much engrossed with the body. That is called materialism. But when you understand what is spirit and you act accordingly, that is called spiritualism. So Arjuna was hesitating to fight with the other party because he had bodily relationship with them. So between Arjuna and Kṛṣṇa there was discussion, but that was friendly discussion. Therefore, when Arjuna understood that simply friendly discussion could not solve the problem, he became His disciple. Kṛṣṇa... Arjuna surrendered to Kṛṣṇa, śiṣyas te 'ham śādhi māṁ prapannam: (BG 2.7) "My dear Kṛṣṇa, so long we are talking as friend. Now I become Your regular disciple. Kindly save me by instruction. What I have to do?" Therefore, when this stage was arrived, Kṛṣṇa is advising Arjuna as follows: śrī-bhagavān uvāca. Now, it is said here... Who is saying to Arjuna? The author or the recorder of Bhagavad-gītā... Bhagavad-gītā was spoken by Kṛṣṇa. It was a discussion between Kṛṣṇa and Arjuna, and it was recorded by Vyāsadeva, and later on it became a book. Just like when we speak it is recorded and later on it is published as a book. Therefore in this book it is said, bhagavān uvāca. Vyāsadeva is the writer. He does not say that, "I speak." He says, bhagavān uvāca, means "The Supreme Personality of Godhead said."