Sforno on Exodus 21:21/ספורנו על שמות כ״א:כ״א

1כי כספו הוא, and it is up to him to discipline him. Sometimes a slave’s insubordination continues until the master is forced to administer a strong blow. We have read of such in Proverbs 17,11 ומלאך אכזרי ישלח בואך מרי יבקש רע, “an evil man seeks to rebel; therefore a cruel messenger will be dispatched against him.”