Garnet Defying God

Looking at it from that perspective, the thing that changes about him at his core isn't his willingness to do whatever needs to be done to get home. It isn't the giving up on mercy. It's turning his back on his friends' memory. Which he does again in Thunder Bringer when he chooses to let his men die. That is the moment where he stops being who he was. And while it on the surface looks like he is changing because he became ruthless, if that was the case, then you can say that different beasts is the point that he changed. And the reason it's Thunder Bringer and not Scylla is because of Eurylochus.

In Scylla yes, he sacrifices 6 of his men, or as Eurylochus says 6 of our friends. But despite having spent well over a decade with these men, at the end of the day, they are still soldiers whose job is to die for their king. They don't have any deep connection to him then that really. But in Thunder Bringer It is Eurylochus who he has to face when he chooses to let them die. It is one of the people closest to him. And to make that choice he has to let go of his friend's memory.

I know this is all just a headcannon, and the canonical reason he changed is because of him abandoning mercy. But one last thing that I have that points to why it was no longer honoring his friends memories that made him a different person is WYFILWMA. After he confesses all the horrible things he has done. The horror and bloodshed he caused. She tells him that he is still her husband. Now I know this is because there isn't a way that he could change where she would no longer love him and that's the point of the song but I couldn't help but think about the fact that Polites isn't mentioned. In the song where he is told that he's still himself. Maybe it's because nothing he says in the song is the reason he has changed.