Writing in his weekly column on News24.com, Mr van Tonder described how he showed the graphic video to journalism students as part of a debate on the ethics of dealing with sensitive material.
The popularity of the video demonstrates the negative side of uncontrolled information available on the web, said Mr van Tonder.
"This type of material is not produced by professionals and not distributed by traditional media. There is no regulation and no code of ethics," Mr van Tonder told dotJournalism.
Students thought they had been desensitised by violence in films and by gang violence seen in South Africa, but there was a 'shocked silence' in the room after the Nick Berg video had been played.
Only three or four students decided they would have used the footage. Some said they would show edited scenes without the especially disturbing soundtrack.
Mr van Tonder himself refused to watch the video.
"I'm a former conflict photographer and soldier but I've moved on - and I don't want to be exposed to this," he said.
"But these students will be working as editors in a few years' time and will have to deal with this kind of issue - they need to be prepared."
Nick Berg was a 26-year-old American contractor working in Iraq. He was captured and beheaded by a group known as Al-Ansar which published a video of his execution on its website, Al-Ansar.biz.
Most news organisations published only a still from the video, but the footage was widely available on other websites and 'Nick Berg' was the top search term on Google in the middle of May.
More news on dotJournalism:
American beheaded online
Beheading video site pulled
See also:
Johann van Tonder’s column on News24
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