Framing them as a cunning predator that can strike mysteriously and who knows where the next attack vector is, well, that's just feeding the paranoia - and playing into their hands.
Besides, if your goal is to react as encouraged above and remain as dispassionate as possible, framing them as mentally ill helps put you in a mood of pity (or even compassion), whereas framing them as an animal suggests that they must have battle done to them.
EDIT: Not to mention just the basic humanity of seeing humans as people who can have problems, instead of simply writing off people wholesale if they take a wrong turn.
But it's true, in a way. You can't really assume much about the limits of a person who's willing to take their own life down to assume control over yours.