Krishna in order to persuade him (ARJUNA) to fight, reminds him that there is no such act as killing. He explains that the "atman" or soul is the only reality; the body is simply an appearance, its existence and annihilation are illusory. And for Arjuna, a member of the "Kshatriya" or the warrior caste, fighting the battle is 'righteous'. It is a just cause and to defend it is his duty or dharma.
"β¦if you are killed (in the battle) you will ascend to heaven. On the contrary if you win the war you will enjoy the comforts of earthly kingdom. Therefore, get up and fight with determinationβ¦ With equanimity towards happiness and sorrow, gain and loss, victory and defeat, fight. This way you will not incur any sin." (The Bhagavad Gita)