This is a good question. William Golding's Lord of the Flies is set on a tropical island and the characters are all English schoolboys. Piggy is the most intellectual boy in the group, and because he is committed to using logic and reasoning, he becomes a direct adversary to Jack. Jack has no use for anything but hunting and following his own rather cruel and savage instincts.
This animosity is evident from the very beginning. Piggy is just trying to be sure he has gotten everyone's names, but Jack rudely interrupts him:
“You’re talking too much,” said Jack Merridew. “Shut up, Fatty.”
In the end, after being tormented, taunted, and ridiculed throughout the entire novel, Piggy has had enough. When Jack steals his glasses, leaving Piggy virtually blind, Piggy makes Ralph take him to confront Jack. Simon has already been killed, and Piggy has to know that Jack is capable of ensuring that Piggy is murdered, as well. But he no longer cares.
Piggy is determined to control his own destiny, and if he is going to die he wants to die with the conch in his hands, asking the right questions:
“I got this to say. You’re acting like a crowd of kids.” The booing rose and died again as Piggy lifted the white, magic shell.
“Which is better—to be a pack of painted Indians like you are, or to be sensible like Ralph is?”
A great clamor rose among the savages. Piggy shouted again.
“Which is better—to have rules and agree, or to hunt and kill?”
Perhaps he hoped he might get through to Jack and his savages, but of course we know the answer comes in the form of a boulder which smashes both Piggy and the conch.
Piggy has always known that Jack would kill him if he could, so to that extent, perhaps Piggy is at least partially responsible for his own death. I prefer to think that Piggy is responsible for choosing the time and place of his own death, knowing he could make one last stand and hoping it would be enough to sway the others. Jack and Roger are literally responsible for dropping the boulder which killed Piggy, but Piggy is probably responsible for being where he knew they could kill him. Piggy knew he was going to die and wanted his life to count for something.