Every protagonist needs a good antagonist. The novels I like best are the ones where you don’t even know who the antagonist it, at least not at first. Kind of like a Marvel comic’s adaption to novels, the antagonist is your best friend, or your roommate, or someone else, where having them be your enemy means something.
In real life it’s the same. If someone tells you to go f**k yourself, you might get upset, but if it’s your husband? Then it’s a whole new ball game. The ones closest to us are the ones most capable of hurting us. They know just which targets to aim at.