Any thread on dolphins teeming with the usual tripe about their intelligence, playfulness, peacefulness, and near parity (if not outright superiority) with humanity is not complete without mentioning their tendency to commit infanticide and kill other marine mammals for fun:
http://www.telegraph.co.uk/earth/earthnews/3323070/Killer-do...
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1689180/