Moral Event Horizon: Ratman approaches the final line several times before he finally crosses it with murder.
To the rats, he crosses it when he systematically traps and drowns all the females and pups that'd stayed behind when he took the rest to his workplace to kill Jones.
Tear Jerker: Crispin Glover's entire performance in the 2003 movie, at least when he's not Adorkable or doing something nasty. To name a few examples, his breakdowns at his mother's funeral, and when Frank Martin fires him. Also, his reaction to the death of Socrates shortly after he was fired.
The Woobie: Ratman but more so in the movies than in the book. Crispin Glover's take in particular. His fall into madness is really something to see, and his woobieness is just something that he alone could pull off.