Why is "Zevach Resha'im To'evah"?
Sha'arei Teshuvah (4:2): Without Viduy and Teshuvah, a Korban Chatas does not atone.
Malbim: Even if a Rasha brings a Korban with a full heart, as long as he is a Rasha, i.e. did not repent from his former evil, it is despised. It is like the king's enemy who brings a gift to him. He despises it, even if he brought it with all his heart, since he hates him.
What is the meaning of "Af Ki v'Zimah Yevi'enu"?
Rashi: All the more so his Korban is an abomination when he brings it for evil and with evil counsel, e.g. the Korbanos of Bil'am and Balak, who brought Korbanos in order to curse Yisrael.
Malbim: All the more so his Korban is an abomination when he brings it with Zimah, i.e. his thoughts are sunk in sin, also at the time he offers it. Aside from not accepting the Korban due to the one who brings it, it is not accepted due to the bringing itself, which is for the sake of evil.
