He had wanted to teach the teen blogger a lesson.
Neo Gim Huah, 49, was offended by Amos Yee Pang Sang, 16, posting about Singapore's founding father Lee Kuan Yew in his blog.
"I felt as if it was my relative being insulted," said Neo, who is self-employed and a father of three.
He thought that slapping Yee would instil fear in the teen and "let him know what are the ways of the world".
Yesterday, Neo, who had slapped the teen outside the State Courts on April 30, was sentenced to three weeks' jail.
The prosecution had initially recommended two weeks' imprisonment for Neo's single charge of voluntarily causing hurt.
But District Judge Ronald Gwee upped the sentence to three week's jail.
Said Mr Gwee: "A strong message must be sent to the public at large and particularly to like-minded persons as the accused (Neo), that his brand of vigilante justice must never be allowed to take root."
In his oral remarks, Mr Gwee mentioned there were several aggravating factors, like Neo's premeditation.
He found it disturbing that Neo had chosen the time and place and decided to confront Yee at the walkway between the Family Justice Courts and the State Courts where he was able to "maximise the publicity".
After slapping the teen on his left side of his face, Neo had shouted: "Sue me, come and sue me!"
Neo's identity was established the next day, May 1, when CCTV footage of the vicinity showed him leaving Chinatown Point in a red Honda.
In his submissions on sentencing, Deputy Public Prosecutor Winston Man said: "The accused (Neo) admitted that it had crossed his mind to slap the victim (Yee) in the course of the victim's daily reporting to Bedok Police Division. However, he chose not to strike at that location as no member of the media was present there."
In his mitigation plea, Neo said he realised he had committed a great mistake.
Unfortunately, on the day of the incident, he said he couldn't control himself when he saw how arrogant Yee had looked.
The teen blogger himself faces two charges - one of transmitting an obscene picture electronically and another of uploading content online which contained remarks against Christianity.
Pleading for leniency, Neo, who spoke in Mandarin, asked to be fined instead of jailed.
He said in a written note to the judge: "What I feel (about Yee), is the same as everybody's feelings.
"This child is so disobedient, even his parents, police, the courts and society wouldn't have any impact on him."
Neo also apologised to Yee and his parents in the note.
This article was first published on May 12, 2015.
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