Not usually.
The common name “killer whale” originated by watching the way these magnificient animals work when hunting for prey. They typically hunt in well-organised packs and they strategise. They are also the largest of the meat-eating/hunting whales. Unusually for whales, they will eat other whales, including dolphins, or calves of the larger whales (like Sperm whales and California Gay whales).
They are also quite capable of aggression towards each other. Males fight for females and even females will fight for dominance in the group.
They also have big teeth, which is kind of scarey in itself.
However, documented acts of aggression towards humans by orca (the preferred name for fans of the species) is relatively rare. That said, it has happened.
There was a recent (November 2006) incident in San Diego, California, at the Sea World Park. A Killer whale in a regularly scheduled show grabbed his trainer by the leg and held her down at the bottom of the pool for a minute — twice in a row.
Experts speculate that whale was merely having a grumpy day. It was back at work in performances within a week.