Some characters are just born bad. Serial killers, werewolves, bankers—evil is in their blood and they are driven by a compulsion to do terrible things.
But not all antagonists are out and out villains. Just because your mother stops you doing anything fun, interferes in everything you do and guilt trips you into giving up your exciting plans to go curtain shopping with her, does that mean she’s a psycho who can’t be stopped? Hmm, okay, bad example.
My point is while there are some types of characters whose motivations don’t need to be explained because they are basically insane and can’t help themselves, most of the time the person acting against your hero needs their own reasons for pursuing their goal in such determined fashion.