You want to know what the most upsetting thing about this scene is for me (besides the scene itself)?
It’s the fact that this is something Stiles Stilinski legitimately fears. This is a situation he has probably imagined, words that he’s probably told himself before, and tried to convince himself of during his personal moments of grief. In a way, at one point after his mother’s death, I’m sure he did try to blame it on himself - despite how illogical it was. Which he knew too. Whatever his mother died of it really hadn’t been his fault, but in the back of his mind he probably couldn’t help but think, “What could I have possibly done to deserve something like this?”
He probably spent many nights coming up with the most ridiculous things and trying to convince himself that that was the reason why his mother was gone. And to make it all worse, it’s heartbreaking to see that the biggest fear for Stiles is hearing his father, drunk and full of hate, spew it towards him and repeat the very words he had already told himself numerous times. A man he looks up to and loves incredibly, accusing him of being the one who killed his mother - and then going on to say that he was killing him too. The thing that probably makes this scene so emotional for him is because it’s his father saying it.
The only person in his life who could truly say those words and have Stiles take them to heart.
The reactions and emotions on his face say enough. This is his worst nightmare come to life - this is something he buried for a long time and belittled himself over for simply even thinking about it. So when it’s finally played out in front of him; he takes it. He lets his father say those words to him and acts as though he’s already heard them before… because he has. It upsets me even more to think that this is a moment in his life that he’ll never forget. He’ll never forget the expression on his father’s face. Never forget the words spoken to him. Never forget the tone of voice and build up of emotions lining every accusation. Won’t forget the quickening beat of his heart when his father throws that alcohol bottle towards him. It’s probably burned into his mind forever, and it upsets me that that is something he has to live with for the rest of his life. Regardless of the fact that it wasn’t real, those emotions he felt were, and that is more than enough to leave lasting damage.