This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Ginger Snaps
Brigitte killing Ginger at the end of the first movie. Even worse is Brigitte's reaction.
Throughout the first movie, Ginger and Brigitte's relationship continueusly falls apart. At the begining they where the best friends they could have and by the end Brigitte stabs her in the heart.
Here is one example of their arguments.
Brigitte: You want me? Then stop hurting everybody else and take me, take me!
Ginger: I don't want you! I don't even know you!
Ginger: We're almost not even related anymore.
Ginger: I said I'd die for you !
Brigitte: No, you said you'd die with me because you had nothing better to do.
In the second film, there is possible fate that awaits Brigitte after Ghost's betrayal. Here's the idea; she is being kept in a dark basement and is likely going to be forced to kill whoever Ghost feeds her. And in this series it's commonly believed there is no going back from a transformation. And cronologically speaking this was the last film, so it might just be Brigitte's final fate.
In the prequel, Ginger finds the werewolf who bit her (who was just a little kid) crying at his mother's grave and hesitates to kill him, instead she talks to him (doubles as a Crowning Moment of Heartwarming) saying "I'm sure she misses you too". This is followed by the boy being captured and being forced to die at his fathers hands.
Also from the prequel, there is the legit fear Ginger has when she thinks she will end up killing Brigitte.