If you had asked Ariel if he truly hated someone enough to wish for them to be incinerated by the sun a year ago, he would have said no.
Although he hated the Crouch family, it wasn't to the point of wishing them death, just that they would leave him alone forever since they had ruined everything his parents had left him.
But ever since he met William, not only could Ari honestly say he hated somebody to death, he also wished every terrible thing in the world to happen to that person and only that person.
Each opportunity that he had, William showed him that he was just an all-around terrible person, and Ari sincerely wished that somebody would humble him and just get him out of his life.
So, although it wasn't the brightest thing to do in front of everyone and would probably not work, when presented with the opportunity to openly beat William up and possibly get him off the show, Ari took it.
