It's easy to say that from the sidelines. I think it's a small price to pay for maintaining your sanity while its under siege by a mentally ill person. So long as you're not harming anybody, who cares how you frame your aggressor?
Framing them in an appropriate way is significant. Framing them as a mentally ill person can help you cope with what's going on, because it helps you understand what's happening. And while it's not nice, humans deal much better with things that they think they understand (cf: origins of religions)
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.
"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."
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.
