The scar is the area of land on the island on which the plane carrying the boys landed. The plane itself was carried out to sea and only the damaged area was left.
Ralph is perhaps innocent of intentional wrong, but he did some things that could be interpreted as wrong. For instance, he did not stick up for Piggy from the beginning, thus inadvertently allowing Piggy to become a target for bullying - and in that way, somewhat responsible for Piggy's death.
He also, as leader, had the responsibility of organizing the boys' society and making sure the rules were followed. He gave Jack power, and Jack quickly recognized that Ralph was not a true leader. He meant well but he wanted to be liked, whereas Jack had neither of these qualities. This allowed Jack to feel no guilt about breaking rules and hurting others. Ralph's inability to control Jack was a catalyst for Jack's rise in power, and the subsequent abuse of most of the other boys.
Ralph also took part in the killing of Simon, although this was at night and was certainly not planned. He obviously felt guilt, but that does not absolve him of responsibility.
These are just some ways Ralph is not innocent...but he IS innocent, for the most part, of doing things with the intention of hurting anyone. He simply wasn't a leader - just thrust into that position because he had the conch when the initial meeting was convened, and this was only because Piggy recognized the conch's potential as a unifying device.