My opinion is that some crimes are forgivable. I know a young man who got drunk and beat someone up. He went to prison. His family and girlfriend forgave him and stuck by him. He has never committed another crime, the couple have just had a baby and he is well on his way to getting a degree. He made a stupid mistake.
Nessling's crime wasn't a mistake. He isn't young and foolish, he's a mature man. He intentionally broke the law knowing full well what the consequences were likely to be. Even so, his crime would have been unremarkable and not worthy of singling out had he not sought to deceive people.
He presented himself as a fine upstanding member of society; an employed family man with a social conscience. He denigrated liars, bullies and child abusers but at the same time he himself lied, bullied people online, and abused the children whose images he viewed and collected.
A crime is one thing, and can sometimes be forgiven. Trust is another thing and is very difficult to regain once destroyed.