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.