Change the definition of Complete Monster according to objective criteria as discussed here, namely

• Moral Event Horizon

• Nonexistent or inadequate Freudian Excuse

Ignore this.

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.