Even if human are capable to kill god, no one can do it, it is a taboo. The reason is unknow, it may be because of the divine aura of the god or it the part of the cultural or psycology. In the past, when Lyu(Ryu) is consumed by vengeance, even when she just massacred an entire familia that have caused the death of all member of her own familia, she can't bring herself to kill the god of that familia. If Lyu can not kill god even in that state, it hard to beleive someone can. It may be because god is the one that control the flow of the soul, kill the god and you may face with terrible consequence after you died.