See also 2 Kings 21:17. Understandably, the writer judges Amon less harsh than Manasseh because the abominable practices of Manasseh were more severe. Add to that the fact that Manasseh, in all his years of reign, had more opportunity to commit evil practices than Amon in his reign, which only lasted two years.
25 Now the rest of the acts of Amon that he did, are they not written in the Book of the Chronicles of the Kings of Judah?