Here it says that that he passed his sons (plural) in fire. Why does it say in Melachim II, 21:6 "Beno"?
Malbim (Melachim II, 21:6): There it says that he passed one son, for he acted to anger Hashem, and one who passes all his children is exempt 1 . Here it teaches that later he passed his other children. To avoid being exempt, he passed his second son by himself. The exemption of Kol Zar'o is when he gives all of them in a way that is liable for, i.e. via the priests 2 . Here it says "b'Gei Ben Hinom", to teach that in every case he was in Gehinom (liable for his actions). His son Amon (whom he did not pass, for he succeeded him as king) was born after Menasheh repented. He was 22 when Menasheh died (33 years after he repented).
Why is "v'Yid'oni" written with a Vov?
'Rashi': This shows that he increased to do much. He added to the image that he made.
Was any particular action to anger Hashem?
Radak citing Pirkei d'R. Eliezer 43: He removed the Heavenly fire, which descended in the days of Shlomo, from the Mizbe'ach.
Malbim (Melachim II, 21:6): All of these abominations of the Kena'ani were to anger Him. There is no Hana'ah from them, and they were not b'Shogeg!
