Piggy, as our surrogate adult, understands that although it was a terrible act, the boys must move on. His willingness to place the blame on Simon instead of everyone else is his way of placing this act hehind him in an attempt to still save civilization.

Ralph has felt the weight of his leadership since the beginning. He has felt an enormous responsibility for the safety and well being of all of the boys. Although losing the birthmark-faced boy early in the novel, this loss truly shows how far the boys have come from "normal". At this point, Ralph feels that he has lost his leadership value, and it will be tough to salvage from here on out.

Both Ralph and Piggy understand, at some level, what has happened to Simon. Ralph tells Piggy, "That was Simon . . . That was murder," and struggles to comprehend the enormity of their action. Unfortunately, reason is not Ralph's strength, and he can only rock back and forth, murmering, "I'm frightened. Of us. I want to go home. Oh God, I want to go home."
Piggy tries to rationalize the death by claiming that it was an accident, that they were scared, and that it was somehow Simon's fault: "He was batty. He asked for it."
If Ralph understands but can't accept, Piggy accepts but hides understanding under a cloak of denial.

How do Ralph and Piggy react to the death of Simon?

At first they are in a state of semi-shock and refuse to accept the reality of it and then they spend most of the rest of the time talking about it in such a way as to try and make themselves believe that it was completely by accident, that the other boys couldn't possibly have meant for things to escalate to the point of his death.  Piggy even goes so far as to suggest that Simon was pretending and is actually still alive somewhere.

Of course they then begin to try and rationalize their own role in it and that they were "just at the outside" and not actually directly involved in killing him.