What is "Tarah"?
Radak: It is the scabbard (case) of the sword. Yonason translates Lidnah; this is like "va'Yashev Charbo El Nidanah" (Divrei ha'Yamim I, 21:27) in Hebrew, and "Eskeriyas Ruchi Ana Daniel b'Go Nidnah" (Daniel 7:15) in Arame'ic. The Lamed in Lidnah is in place of a Nun, just like Lishkah in place of Nishkah.
Why will Tzadikim be cut off?
Radak: Tzadikim die due to the sins of Resha'im. And so it says "Nasnu Es Nivlas Avadecha Ma'achal l'Of ha'Shamayim" (Tehilim 79:2).
Radak citing Rav Sadya Gaon: Tzadik is one who served Ba'al constantly, with all his heart; one who did not serve constantly, they called him a Rasha. Hashem considers both of them Resha'im. Similarly, he warned Lavan not to speak "Im Yakov mi'Tov v'Ad Ra" (Bereishis 31:24). Lavan considers Tov to steal Yakov's property and not kill him; Ra is killing him. Both are evil regarding Yakov.
Radak citing Targum Yonasan: I will exile from you those who merit, in order to destroy the liable.
Malbim: They will be cut off first, lest they see the afflictions of the generation, and lest they protect the generation with their virtue and impede the fire of punishments. As long as the evil is individual, only the sinners are destroyed and the Tzadikim are saved. Then eradication is not decided. After Hashem decreed eradication, Tzadikim are cut off first, and nothing stops the fire.
