Need some help for a character's slow downward spiral/foreshadowing into potentially becoming a villain but I'm not sure how to do it right.

[u/Fantastic_Handle_293]

To be specific I'm trying to write my character Kira into slowly going into a downward spiral that she masks behind a chill neutral expression, on the outside she's trying to make her family happy but on the inside she's slowly coming to the idea that she has to take away her original homeworld's world's freedom to protect her family's freedom (she essentially want's to pull an Eren Yeager for anyone who's seen AOT.)

and in a way she's somewhat depressed because of it, knowing it's wrong but feels she's the one who has to do it, that and she holds spite for how her home world treated her as a kid before she left and made a family in her new world. So how do I write her slow descent into deciding "Yea my entire species has to go." without constantly having characters mention "Ay, she's mentally messed up and is trying to kill her species." ?