Why will "Ikesh Lev" not find good?
R. Yonah: This is one whose Da'as is errant. He does not recognize Emes. One finds good via Da'as - "Maskil Al Davar Yimtza Tov" (16:20).
Orchos Tzadikim (Sha'ar ha'Ratzon DH v'Acharei): If he hardens his neck against those who rebuke him, and one who shows to him the good path, [he will not fix his way]. Rather, he should accept their words with desire.
Malbim: This is one whose heart (the power that rules in him) goes on crooked paths, and not in the way of Chachmah, which is the proper path. Even though he seeks the good path based on his Sechel, he will not find good, for the good path is only via Chachmah. One can find Chachmah only via receiving it from Chachamim. However, it is possible that he will not do evil.
Why will "Nehpach bi'Lshono" fall in evil?
R. Yonah: One who recognizes the Emes, and his lips vindicate a Rasha because he loves him, and he condemns a Tzadik because he hates him, he will fall in evil, for he is worse than Ikesh Lev. He intentionally helps Sheker. Therefore, it says that "Ikesh Lev" will not find good, and "Nehpach bi'Lshono" will fall in evil, which is worse.
Malbim: The tongue represents speech with Binah. One whose tongue speaks Tahpuchos - deceiving comparisons and false investigations in Elokus and Emunos, he goes out to heresy. Not only will he not find good - he will fall in evil, for "Shuchah Amukah Zonah" (23:27), i.e. heresy.
