Reference for Avodah Zarah 30:14

ואי דלא קטיל אמאי לא לעולם דלא קטיל והב"ע במשמוטא דזימנין דעביד לאצולי נפשיה

If [on the other hand] he has never committed murder, why not [sell them to him]? It refers indeed to one who has not committed murder; but we may be dealing here with a sneaky thief who is apt at times [when caught] to save himself [by committing murder].

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