I realize that this idea may have been done before, but why not make the monster have some sort of extra sense that is attuned to something humans emit with emotional outbursts. Perhaps the creature is not overtly big, but big enough. Say no bigger than a T-rex at most.
Now say the creature's sense of smell was not only key to most scents, but particularly attuned to pheromones, or chemicals given off by emotional states, whatever the proper researched equivalent is. Perhaps the creature could be drawn in by positive, more passionate emotions, and as such, it more often attacks those happier in life at the moment.
So the mother's depression, and anybody else saddened at anything or the creature, would actually protect them, because the creature could perhaps associate the pheromones of sadder states with decay or sickness [getting metaphorical here].
The more the creature comes, the worst those who are depressed become, even if they are spared, because people who aren't depressed or sad are suffering for no seemingly fair reason.
So the mother has to overcome her depression to get the strength to help fight back against the creature, even if overcoming that depression makes her a target that she's avoided being until then.
As for how the creature's 'sense' is figured out, I'm sure that could be deduced by some outside investigator the mother comes to associate with to some degree.
The climax could be that, after luring the creature in to an isolated location, she relapses into a depression at being near the creature, and death, which ironically spares her. Now she's trapped with a creature who seemingly shows little interest in her, and more interest in getting out. Getting away from the 'sick smell', if you were to dive into its mindset for a moment.