how did piggys death affect everyone? how did it affect Ralph's ability to maintain control?

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Piggy's death was the death of logic, organization and (perhaps) their best chance of ever getting rescued. THe boys are in shock for about two seconds and then they revert back to jeering and acting like savages. Ralph of course is horrified. Ralph had already lost control well before Piggy was crushed. Piggy's death merely completely isolates him from all the other boys.

so nobody really killed him?

Yes, Roger levered a big rock down a cliff. The rock crushed Piggy.