Orcas are the largest members of the dolphin family. They used to be called killer whales, until save-the-whale groups decided that "killer" was mean and "whale" was incorrect. However, both are actually true.
Back in the day, when humans were busy killing whales for all that nice blubber and whalebone, they noticed these other smaller whales also killing whales, presumably to eat them. The human whalers called them "killer whales," because they didn't have much imagination. Over in the Pacific Northwest, the Native Americans had known about these creatures for a while, and thought they were the gods of the sea, with sea lion slaves and houses under the ocean. They had a lot more imagination, and also respect for the Killer Orca Whales.
Being small for a whale, orcas weren't hunted commercially, which saved them from being turned into whale-oil lamps and corset stays like the big boys. They had a few problems later when humans switched from hunting whales to commercial fishing, and it was decided the orcas were eating too many fish. However, killer whales are hard to catch and kill, and fortunately for them, the save-the-whales movement began at just about the same time and it became more fashionable to not kill the pretty orcas and use the fishing boats to go out and watch them killing fish instead.
Places like Sea World were created first to display dolphins and orcas, and then when humans became more enlightened, to discover dolphins were actually very intelligent and we shouldn't be keeping them cooped up in swimming pools. Humans found themselves in a bit of a pickle, because the only way to study an animal that can dive fifty meters and swim a hundred miles a day in 30 degree water is to keep it cooped up in a swimming pool warm enough for a human to survive in. The humans tried to mitigate the whales' discomfort by not "training" them to do weird tricks, but only encouraging them to display "behaviors" that were natural.
The other problem was that this stuff wasn't cheap. A single orca needs about 500 pounds of fish and squid a day, so Sea World needed one big-ass sushi bar for their whales. That meant they had to keep having shows to bring in people who would buy stuffed plush Shamus; but tourists wanted to see the orcas jump out of the water and splash stale pool water all over their kids. So the killer orca whales and their dolphin buddies had to keep performing their natural behavior tricks. Not surprisingly, some of them got pissed off.
One who keeps getting pissed off is Tilikum. Tilikum has killed three people so far, and he's just getting warmed up. Although the official Sea World policy is that the bull orca is just "playing," the reality is probably that the huge beast is mad as hell and wants to kill something. Tilikum has never been trained to "play" with people, and is kept by the facility because he is an expert breeder--a bull in more ways than one. There's a reason they were never called "fluffy bunny whales" and Tilikum is a perfect example.