The problem with this article is the intial assumption: "the normally placid dolphin". Dolphins are always given the image of being fun-loving, caring animals that like nothing better to help others. This just isn't the case. Sure, they are friendly and helpful sometimes. And other times they're vindictive and vicious. Torturing baby porpoises is fairly extreme, but this is by no means the first example of dolphins being nasty.
http://www.newscientist.com/article/mg16321975.200-unusual-suspects.html
http://www.newscientist.com/article/mg16321975.200-unusual-suspects.html