I believe that Seth Norris was killed by Smokey for two reasons. One reason was internal (story-driven) and the other was external (production-driven).
The simpler explanation is the external, production-driven reasoning behind killing the pilot.. in the pilot. The attention-grabbing magnitude of a man being ripped from the cockpit by a loud, large, and looming unseen monster is hugely captivating; it jolts the audience and makes them wonder what the hell that thing could possibly be. I truly believe that a lot of the details in Lost were worked out after the story had been set. In other words, the writers had no intention of or idea to introduce Frank later on as the original pilot of 815. So the idea of candidates in this sense is far from realized in season 1, which explains why Locke is dragged-off by the monster in Exodus. They didn’t have the idea of candidates until season 4 or 5, so the idea that the monster wasn’t allowed to kill certain folks was long from conception.
Secondly, I believe that, within the context of the story, the monster killed Seth to send a message to the other survivors. Again, within the context of the story, accepting that Smokey was operating under “candidate rules” even in the Pilot, he was not able to kill Jack or Kate (not sure about Charlie). So he killed Seth, who was not a candidate, to scare the hell out of the other Losties.
KEEP THE THEORY CONTESTS GOING!