Rashi on Deuteronomy 13:10/רש"י על דברים י״ג:י׳

1כי 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).

2ידך תהיה בו בראשונה THINE HAND SHALL BE FIRST UPON HIM [TO PUT HIM TO DEATH] — He who was to be seduced to idolatry has the duty to put him to death; if he did not die by his hand he must die by the hands of others, as it goes on to state “and afterwards the hands of all the people” (cf. Sifrei Devarim 89:8).