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