Perhaps not a monster, but in "It takes two", there is a particular scene where you have to murder a friendly stuffed elephant to get your in-game daughter to cry.
I have been playing video games for decades at this point but that one really shook me up. You pretty much execute a toy begging for its life. As soon as that scene was over it genuinely took me a few days to come back to it.
That was a hard rewatch: https://youtu.be/12FNU8bNEbE?si=BKZCynsHhoz5GN2m&t=65
The ending credits show her patched back up with needle and thread, but they know what they did.
https://www.youtube.com/watch?v=0bnKr0M1Tzs
Long time gamer here too and this also really got me. I'm not sure exactly what it was; perhaps a turn to a darker tone where I wasn't expecting it. For all the interpersonal disagreements the game is generally 'whimsical.' It was the desperation of the elephant that really got me ... the bargaining, the confusion.
I find that as I get older I respond more to media like this. I'm not sure if it's emotional intelligence, being more present, or something else.
[dead]
I’m apparently a heartless monster or something, but that elicited less than zero for me. I wanted it to end because the squeeky annoying voice and over the top cuteness was for effect.
I’m happy for you that you felt that was dark. Well, IDK, maybe I’m happy for you, hard to say.