One of the strongest lines of evidence that I have noticed is how Jack had responded to Simon's death, his form of dress, and his brutality towards his tribal members. After Simon has been killed Jack shows no remorse or guilt. Instead, Jack uses Simon's death as a way to bond his group by telling hem that the beast had snuck in on them and had been disguised and would return again.
When Jack asits before his tribe he now ornated in his red facial markings and sits like a ruler. He holds himself up verbally and physically above the others.
He has his people tie up Wilifred for reasons he nor they even understand and allows or rather has the boy beat up just because he had become upset.
Piggy believes he has the right to be heard because he holds the conch. However, Rodger lunges a large boulder at him and it sends the conch crashing and destroyed and Piggy over the cliff smashed and broken and swept out to sea.