A relative should not be killed even if his wrongdoing warrants capital punishment. Rather, he should be thrown out of the family. Since he has already been killed by his own sin, why kill him again?
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Expressions researched:
"A relative should not be killed even if his wrongdoing warrants capital punishment. Rather, he should be thrown out of the family"
Srimad-Bhagavatam
SB Cantos 10.14 to 12 (Translations Only)
SB 10.54.39, Translation:
(Again addressing Kṛṣṇa, Balarāma said:) A relative should not be killed even if his wrongdoing warrants capital punishment. Rather, he should be thrown out of the family. Since he has already been killed by his own sin, why kill him again?