Rashi on Deuteronomy 19:13/רש"י על דברים י״ט:י״ג

1לא תחוס עינך THINE EYE SHALL NOT PITY [HIM] — i.e. that thou shall not say, The first (the one) person has already been killed, why should we kill this also, so that two Israelites will be killed? (Sifrei Devarim 187:7).