Why did the boys kill Simon?

The boys killed Simon because their minds were so stuck into the song they were singing "Kill the beast! Cut his throat! Spill his blood!" that they weren't really focusing on anything else when Simon stumbled into the middle of their horseshoe they immediately figured that it was the beast and with them singing the song they followed exactly that.

Describe the event in chapter 8 that truly shows the boys descent into total savagery.

The event in Chapter 8 that shows the boys have become total savages is when Jack and his hunters go hunting for another pig to eat. Jack now has become so familiar with hunting he now know how to creep up on the sow without it hearing him. Also when the boys finally got hold of the pig they all hurled themselves at her and Jack continued to stab her and ended his killing by slicing the neck. The boys no longer are afraid of killing they have become accustomed to it.

How is Simon different from the others?

Simon is different from the others because when it came down to the boys deciding who was going to go back and tell Piggy that they were staying on the Mountain until morning time Simon volunteered himself to go while the rest of the boys were scared to go by themselves. Simon seems like the only one that still has a little bit of hope left in him and he is willing to take risk in doing anything to help Ralph.