רמב"ן — ועשיתם לו כאשר זמם - ולא כאשר עשה

Q — עדים זוממים are executed only "כאשר זמם ולא כאשר עשה" — only if the plot was DISCOVERED before the accused was killed. If they succeeded in getting him killed, they go free. This seems backwards: a קל וחומר should make them MORE liable when their scheme succeeded. Why the opposite rule?

A — Ramban's hashkafic chiddush: בית דין is a divine venue, not just a human institution. If a second pair of witnesses came in time to discredit the first, that very arrival was Hashem's protection of an innocent man — "כי לא אצדיק רשע." Conversely, if the accused was already executed, we must conclude in retrospect that he was guilty: Hashem would not allow "השופטים הצדיקים העומדים לפניו" to spill innocent blood — "ה' לא יעזבנו בידו ולא ירשיענו בהשפטו." בית דין operates with a divine guarantee against מיתת צדיק. (This is why dayanim are called "אלהים" — see Ramban משפטים כא, ו.)