What did he see?
Rashi: He saw in his heart that it is not good to do so.
Malbim: His Sechel saw that his father's way is evil and bitter, and he took Musar not to do so.
Why does it mention a Tzadik Ben Rasha Ben Tzadik?
Radak (6): This corresponds to the generations about which they were complaining. Chizkiyah was a Tzadik. He did not bear the sin of his father Achaz. Menasheh ben Chizkiyah was a Rasha murderer ? "v'Holid Ben Paritz Shofech Dam" (10). Chizkiyah's virtue did not help him. He was punished for his evil; Melech Ashur's officers captured him, fettered him and took him to Bavel. Menasheh's son Amon was a Rasha. He was punished for his evil and his father's; he ruled only two years. His servants rebelled against him and killed him in his house. Amon's son [Yoshiyah] saw his father's sins, and did not do like them. He did not die for his father's sins, since he was a Tzadik.
