A dart is shot from a gun - same as a bullet. I imagine if they said that it was killed humanely then a marksman would have been used to shoot it. It could also have been trapped with food, as I imagine it would probably have been hungry been kept penned up in the zoo, and was used to being fed. PETA, the animal rights organisation, advocates the humane killing of animals which are kept in captivity, because they maintain that the animal is better off dead than being unfree. I don't agree with that, although I agree with their other policies of ethical treatment for animals. It is a shame that the poor creature was killed.