I think you're dramatically underestimating an orca's intelligence. Orcas play with TOYS by dragging them down under the water--they don't play with each other that way, and they don't ordinarily play with humans that way, either. The Orcas kept by Sea World are fish-eating culture orcas--they're not mammal eaters. They were not, at least, crazy enough to try to work with animals that have learned from birth to hunt sea lions and whales. So at no time in this animal's history was he taught to grab living things off of land and drag them under the water.
Orcas tend to be very gentle with humans--they can see how small we are, and they realize we must be fragile, just like a baby orca. This reaction to our mammalian nature, and our relative helplessness in the water, is most likely the reason why dolphins have occasionally rescued drowning people--they push them to the surface the way they would help an ailing dolphin.
What has never ceased to amaze me is why attacks on humans by cetaceans are so rare. I wonder what it is about humans that makes them decide to treat us gently rather than treating us the way they would some other sea mammal.
Tilicum wasn't behaving normally--the agitation of the other animals seems clear enough indication that he wasn't just in a playful mood. This is the behavior of a dolphin trying to harm or kill another dolphin--they will grab it or swim over it, drag it down, and then keep pushing it down to try to drown it. They're perfectly aware that we need air.