Rashi: A Tzadik is saved and a Rasha comes in place of him [to bear the affliction], like Mordechai and Haman.

R. Yonah: A Rasha is the opposite of a Tzadik. Boged is the opposite of Yashar. When there is a decree against a Tzadik and a Yashar, when they give ransom for their souls, [the decree is fulfilled on] another person - "v'Eten Adam Tachtecha", "Nasati Chafrecha Mitzrayim" (Yeshayah 43:4, 3) - Hashem puts ransom for him someone opposite to him.

Malbim: Tachas is [a substitute] the same type as what was liable - Nefesh Tachas Nefesh. Kofer is not the same type as what was liable - he gets atonement via money. When Hashem judges the world, He judges between the Tzadik and the Rasha. One who is condemned, his death will atone for the Tzadik. Tzadik and Rasha are relative terms regarding this judgment. Also if the Tzadik is liable, he will get Kaparah via the Rasha. Boged is Tachas Yesharim. Yashar is not relative; he himself is straight, and he does not need Kofer or Kaparah. Only in worldly matters, it can be that the Boged wanted to kill Yesharim, and the Boged dies in place of them and they are saved - "Tzidkas Yesharim Tatzilem uv'Havas Bogedim Yilachedu" (11:6).