The account given about the life of Manasseh in II Chronicles 33 differs from the parallel account in II Kings 2, in one very important way. Chronicles records that Manasseh repented in the end and was reconciled to God. As unimaginable as his evil was, he cried out to God and was answered and restored.
This represents a problem for many people when it comes to his forgiveness, considering the extreme depth of his evil, which included sacrificing children to demon gods. The problem people see is how can God forgive such a person? The question demonstrates the problem, in that those who cannot accept this think that there are some sins which cross a line, not knowing that any sin is a disqualifier.
God puts no price tag on sin, all have sinned and fall short, as Romans 3:23 tells us. All mankind is broken and doomed, if not for His forgiveness on His terms. How this is better understood is that it teaches us that God can save all who call upon Him, should they so choose.