This story is true.
The carp were put into the moat around the Polar Bear exhibit to eat algae that was growing inside the moat. For some reason, zoo officials had forgotten that Polar Bears are predators, and have an instinct to kill.
Knut is distraught that he is now an ungainly adolescent, and that there is a new baby Polar Bear at the zoo named Flocke that is now getting all of the press attention.
So this is probably just your typical adolescent acting out in order to get attention.
Knut should be happy, though, as the zoo got Flocke ( a female ) so that Knut will eventually have someone to mate with, and make yet more baby Polar Bears.
Here is a link to the story that BBC news did about this:
http://news.bbc.co.uk/2/hi/europe/7336092.stm.