Kommentar zu Dewarim 13:10

כי הרג תהרגנו BUT THOU SHALT SURELY KILL HIM — and if he has left the court after having been acquitted have him brought back for condemnation if you know anything against him; if, on the other hand, he has left the court after being found guilty do not bring him back to plead in his favour (Sifrei Devarim 89:6-7).

כי הרג תהרגנו, the word כי means אלא, but, i.e. but be sure to execute him.

כי הרג תהרגנו, “you are to surely kill him.” Seeing that the words of Torah all express mercy of one kind or another, it is not possible to understand the words הרג תהרגנו as an instruction to take revenge on the individual who is to be executed. If that were the purpose of the execution the Torah would actually train us in developing a cruel streak in our character; this is clearly at odds with all the Torah wishes to teach us. The consideration underlying the Torah’s insistence is the mercy and loving concern the Torah extends to all those who know the guilty party. If he were allowed to get away with it many more people would follow in his footsteps with terrible results for them. To make this whole point clear, the Torah first wrote: “you shall not be compassionate with him;” afterwards the Torah explains its concern for the people by writing that the execution of such a seducer will cause the survivors to relate to G’d with the proper degree of reverence so as to preclude the need for any executions in the future.