What did he see?
Rashi: He saw in his heart that it is not good to do Shofet
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 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.
