…do not show him compassion… (Deuteronomy 13:9)
If the meisis, who tries to entice other Jews to idolatry, is arrested and brought to trial, don’t testify on his behalf in an attempt to get him off. This, again, is counter to standard operating procedure, as we normally want people to testify in defense of the accused. The meisis really is that bad!
As in the previous few mitzvos, the reason is that the meisis is a pox on society. Freeing him to continue his evil work would make one an accessory to his deeds.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Sanhedrin on pages 29a, 33b, 67a and 85b. It is codified the Mishneh Torah in the fifth chapter of Hilchos Avodas Kochavim. This mitzvah is #20 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos; it is not listed among the mitzvos that can be observed today in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.