Ralph is trying to convince himself, he says this to himself and no one else, that the death of Piggy was indeed just an accident. It was not a murder.
Piggy said the same thing of what had happened to Simon. Ralph grew angry and tried to talk about Simon's death as a murder, but Piggy wouldn't allow it. This parallel between what is said after the two deaths is strikingly important. It may indeed foreshadow Ralph's upcoming fate because of what had happened to Piggy after he tried to explain away a purposeful murder as an accident.
The accident to which Ralph blatantly refers here is Piggy's death. He does this en route to go ad see Jack's tribe. Apparently, he is trying to convince himself that he isn't going to be hurt or killed just for going to see Jack's tribe.