Where in that passage does it say God killed an animal? If God can fashion Eve from a rib, Adam from dust, and the whole world from waters, he can surely create skin without needing to kill an animal. Re-read the text more closely. Also God made it clear that our disobedience is what brought death into the world (Romans 5:12), so even if an animal did die for the skin, it was due to our actions, not God’s. God even warned us in Genesis 2:17 that our disobedience would result in death entering the world.