Why does Will tell Dani he needs the gun in Chapter Three? Why does Will explain The Rules to Dani?

Will tells Dani he needs a gun because Shawn was killed by the Dark Suns, and that it has to be Riggs who did it. Will explains The Rules to Dani so she will know that what he is doing has a purpose. He also wants her to know that he would have done the same for her if he had known The Rules when they were kids.