I want to say the jungle symbolizes death because Simon and Piggy were killed there, but neither of them were actually killed there. Piggy was killed on the rocks and Simon was killed on the beach. How can I say the jungle symbolizes death?
The jungle is basically an unknown, as is death. When the boys first arrive on the island, they feel a great deal of fear regarding the jungle and are, at least for the most part, afraid to enter it. As time goes by, the boys seem to become more familiar with, and accepting of, the jungle, as people come to accept their own mortality as they grow older. The characters who seem to feel most invincible and have no respect for death also seem most cavalier about entering the jungle and killing other creatures; those who feel comfortable with the jungle environment are those who seem to give themselves over to the dark sides of their natures and become murderers. The darkness of the jungle is symbolic of the darkness of death, both as an unknown (thus frightening) and as evil (murder and insanity).
Perhaps the jungle represents savagery, and the acclimation of the boys to this environment and level represents their decline into savagery. Little by little, the boys regress in their civilized behavior until Jack smears his face to disguise himself as part of this jungle in his attempt to kill the pig and the boys engage in their brutal slaying of this pig, an act that is symbolic of what is to come with Simon and Piggy. [It is no conincidence that Piggy's name is what it is.]
Of course, this decline into savagery and the cruelty and disorder of it represented in the evil character of Roger leads to the deaths of Simon and Piggy. So, the jungle can be viewed as the cause of death as it nourishes the savage nature of Jack and Roger, especially.
In the history of American Literature, the woods/jungle has often been a place fear, the home of the Indians and the Devil. This is most obvious in "Young Goodman Brown" and other Hawthorne stories and "The Devil and Tom Walker," a somewhat similar story. In many ways the jungle is pathless ... you have to find your own way, it's easy to get lost, and anything may happen. A lot of human behaviors "come out" when you are placed in a situation like this, and some of them are pretty ugly ....