Text 59 

yad ahankaram asritya
na yotsya iti manyase
mithyaisa vyavasayas te
prakritis tvam niyoksyati

Translation

If you do not act according to My direction and do not fight, then you will be falsely directed. By your nature, you will have to be engaged in warfare.

Commentary by Srila Prabhupada 

Arjuna was a military man, and born of the nature of the kshatriya. Therefore his natural duty was to fight. But due to false ego he was fearing that by killing his teacher, grandfather and friends he would incur sinful reactions. Actually he was considering himself master of his actions, as if he were directing the good and bad results of such work. He forgot that the Supreme Personality of Godhead was present there, instructing him to fight. That is the forgetfulness of the conditioned soul. The Supreme Personality gives directions as to what is good and what is bad, and one simply has to act in Krishna consciousness to attain the perfection of life. No one can ascertain his destiny as the Supreme Lord can; therefore the best course is to take direction from the Supreme Lord and act. No one should neglect the order of the Supreme Personality of Godhead or the order of the spiritual master, who is the representative of God. One should act unhesitatingly to execute the order of the Supreme Personality of Godhead—that will keep one safe under all circumstances.

Commentary by Sri Visvanatha Cakravarti Thakur

"For me, a ksatriya, the highest dharma is to fight. Being afraid of the sin incurred from killing friends, I do not desire to start fighting."

The Lord scolds Arjuna in this verse. "Your nature (prakrtih) will make you fight. Now you do not respect my words. But when your irrepressible natural desire to fight arises, then while fighting, and automatically killing your elders like Bhisma, you will smile at me!"