The whole discussion about putting this cub down was totally blown out of proportion by the yellow-press he in Germany (especailly by the beloved BILD.
The animal rights campaigner actually never demanded that the cub should be killed but rather wanted to protest in general against how zoos keep animals.
He actually protested against the killing is this particular case that is also mentioned in the article:
Quote Originally Posted by article
Last December, a baby sloth was put down at Leipzig zoo after his mother refused to care for him.
He used the new case as an opportunity to get publicity for the whole issue and also to point out how different the standards for such situations appear to be.

Well, at least the media had a field day and Knut apparently is now not only a ntional celebrity (which he was before this story came up) but also an international one