The Gadarene swine refers to a story in the New Testament that occurs in Mark 5:1-13 . Jesus meets a man who is possessed by devils. Jesus orders the devils to come out of the man. They beg him to send them into a nearby herd of 2000 pigs. Jesus does so, and the swine immediately jump off a cliff into the sea, where they drown.
As Bertrand Russell pointed out, in considering the moral character of Jesus,
"There is the instance of the Gadarene swine, where it certainly was not very kind to the pigs to put the devils into them and make them rush down the hill into the sea. You must remember that He was omnipotent, and He could have made the devils simply go away; but He chose to send them into the pigs."
One might add that it was also not very kind to deprive the swineherd of his livelihood.