I don't know much about Orcas, but apparently they can develop pathologies when in captivity that make them more aggressive.
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Critics claim that captive life is stressful due to small tanks, false social groupings and chemically altered water. Captive killer whales occasionally act aggressively towards themselves, their tankmates, or humans, which critics say is a result of stress. Unlike wild killer whales, captives have attacked and even killed people such as their handlers or pool intruders,[111][112] with nearly two dozen attacks since the 1970s.[113][114] Tilikum, an orca, has allegedly been involved in three fatalities. [115]
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http://en.wikipedia.org/wiki/Captive_orcas