Killer whales are predators of large mammals--unlike other whales, which feed on tiny krill or small fish. So... do they ever eat humans?
A popular video shows a killer whale jumping spectacularly into a group of kayakers. But the event shown is a fake, created as an advertisement for a sports drink.
So far, I haven't seen any information that kayaking where killer whales occur isn't safe. But keep your distance--it's illegal to approach or harrass marine mammals.
Attacks on humans--very rare in the wild
There have been very few confirmed attacks on humans by wild killer whales, none of which has been fatal.
In one instance, killer whales tried to tip ice floes on which a dog team and photographer of the Terra Nova Expedition was standing. There was speculation that the barking of the sled dogs may have sounded enough like seal calls to trigger the killer whale's hunting curiosity.
Recently, a tourist in Antarctica made a remarkable video of a group of killer whales cooperating to capture a seal resting on a small ice floe. The whales carefully eyed the seal, by bobbing their heads out of the water. Then they cooperated to push the floe away from larger floes the seal could escape to. Finally, in a carefully coordinated movement, all the whales rushed at the floe, creating a big wave that--after several tries--washed the seal off the floe.
This incident clearly demonstrated planning, cooperation, and close coordination. In light of this event, the attacks on the Terra Nova photographer above can be seen as normal hunting behavior. So, if you are kayaking in the Antarctic, don't haul out onto an ice floe to eat your lunch--unless you want to BE lunch.
In the 1970s, a surfer in California was bitten, and in 2005 a boy in Alaska who was splashing in a region frequented by harbour seals was bumped by a killer whale that apparently misidentified him as prey."
In 1820, sailors from the wrecked ship Essex were sailing across the Pacific in 25-foot lifeboats. One night, they were attacked by a killer whale, which took a chunk out of one of the boats, then splashed about them for a while. Finally, they drove it off by striking it with oars. Since the attack occurred in the dark, we can't be certain it was a killer whale.
Sometimes, the slaves rebel
"Unlike wild killer whales, captive killer whales are reported to have made nearly two dozen attacks on humans since the 1970s, some of which have been fatal." From Wikipedia. These attacks are probably the result of the stressful, unnatural living conditions for captive whales.
Killer whales are highly intelligent, wild animals--so it's understandable they would object to the slavery of working to entertain humans.
Killer whale kills trainer in "premeditated" attack: Expert commentary.