What aspects of Tilikum early life may have caused him to display abnormal behavior?

Tilikum was captured at the age of 2, and after being shipped to an aquarium in British Columbia, Canada, he lived with two females. He was at the bottom of the social structure, and because he was bullied so often by the two, he was placed in a different tank for protection. This is one suggested cause for his aggressive behaviour.

Another aspect is biological. There are two kinds of whales; resident and transient. Resident whales are whales that live in one area. Transient whales travel all over and tend to be more aggressive. It's likely that Tilikum was naturally transient.