Not to say anything in his defense

This mitzvah forbids offering arguments or testimony in defense of the mesit (enticer to idolatry). Normally, Jewish courts pursue every opportunity to defend the accused, but the Torah makes an exception here. The enticer’s actions threaten the spiritual survival of the community, and therefore, defending him is itself a betrayal of Hashem’s covenant.

Commentary & Classical Explanation:

• Rambam (Hilchot Avodat Kochavim 5:4): One may not seek merits or present arguments in favor of the mesit. His case is judged with special severity.
• Sefer HaChinuch (Mitzvah 466): The root is to prevent even the possibility of softening judgment against one who endangers faith at its core.
• Talmud (Sanhedrin 67a): Uniquely, the court is not permitted to seek acquittal for the enticer, only conviction.