When Arjuna began to drag his enemy, bound in ropes, back to the camp, the All-Attractive spoke with anger in the glance of his lotus-like eyes: “Pārtha, it is not at all right to spare this so-called brahmin! Kill him!!! He killed your faultless children while they dreamt at night! A truly moral person does not kill an enemy who is frightened, without weapons, begging for mercy, ignorant of their mistake, ridiculous, intoxicated, unaware, asleep, a youth, or a woman. One who kills others just to save his own life is shamelessly wicked. Death is good for such people, otherwise their horrific faults drag them ever lower.
“And I personally heard you promise your wife, ‘Pāñcālī, I shall bring you the head of he who you say killed our children!’
“Therefore kill this evil assassin of his own doom who killed your children and defiled his own master. O warrior, a person who has done what he has is nothing but a burnt branch of his family.”
When Arjuna dismantled Aśvatthāmā’s insanely uncontrolled explosion and finally captured him, he did not cut off his head. Why?!? His whole intention in chasing the man was to cut off his head and return it to his grieving wife. Why didn’t he carry out his desire now that he had the chance?
Arjuna could not kill him because was a very saintly and pure hearted man. Such people always naturally feel compassion and pity, even when it is not deserved.
He specifically thought, “This evil wretch killed my sleeping children, but he was just trying to please his master. And after all, he is the son of my guru. It is night right to punish ones superiors, and because he is in my guru’s family, he is among my superiors.”
Kṛṣṇa argues against everything Arjuna is thinking, “Just because someone is in the family of your guru does not make them your guru! Just because someone is born to a brahmin does not make them a brahmin! This man is the worst of all criminals. Look what he did to your children! And he did not even satisfy his master either. You should not let yourself be overwhelmed by compassion for a person who deserves to die!”
As the story continues we will see Kṛṣṇa’s deeper intentions and ultimate suggestion for the issue. But for the present he is putting Arjuna’s compassion to trial.