One example near the beginning is when Jack plans his "hunting face." He paints it black, then looks into the water to see his reflection. He's not pleased with that, so he uses black, white and red and for some reason this new "mask" liberates him. He changes from this point on. Earlier he was excited when talking of hunting, but now he becomes excited and does a little dance. It calls the noises he makes "bloodthirsty snarling."
He also is no longer disgusted by blood. At one point he appears appalled at the blood he wipes on his pants. After key begins killing more often, he is thrilled with the blood. The killing of the sow in chapter 8 is also a very obvious moment when Jack is obsessed with the kill. He has become savage and is no longer cultured after chapter 8.