What is different about the kill of the sow in lord of the flies?

Probably because it is a pivotal moment in the novel. Up until that point Jack and his choir had been merely boys playing at being hunters, filled with schoolboy bluster but shying away from actually stabbing a living thing. However there is a revealing passage about Jack in the section relating to the killing of the sow.... 'His mind was crowded with memories; memories of the knowledge that had come to them when they closed in on the struggling pig, knowledge that they had outwitted a living thing, imposed their will upon it, taken away its life like a long satisfying drink.' From this point onwards Jack no longer has any qualms about killing. Indeed the implication of the passage I've just quoted is that once he'd tasted the power of taking another life, he craved more of that same power.