Musar על שמות 21:20

Commandment 51 provides for the Court to impose the death penalty by the sword in certain situations (Hereg). If a master kills his Canaanite servant by a lethal blow, he is also guilty of execution by this method, viz: (Exodus 21,20) "he shall be avenged." (Leviticus 26,25 states "the sword will wreak vengeance for the covenant" on those breaching it).