It would be an easy mistake to think that the devil wants us all to become ethical relativists. But this is like thinking that the kidnapper wants the child to have some candy. It is like thinking the homeowner wants to the mouse to have a bit of cheese, and so he put some out on a little wooden platform.
The devil actually wants us to indulge ourselves in the delusion that “it’s all relative” because he then wants to accuse us. He is a guilt farmer, and the seed is sin. Relativism is just a doctrine calculated to get fools to throw their seed everywhere.