Emphasize that Complete Monster is a character who is so evil that nothing you say, do, or think will ever help them.
Someone who could kill a sixteen year-old girl and laugh while doing it...very slowly.
The character is not necessarily hated by the viewer. One of this troper's in-progress books involves a group of five contract killers, one of whom isn't just a complete monster, but insanity incarnate. And she's one of the protagonists.