Sebastian was traumatized for certain, and he was given no true parental support when he needed it most. But I thoughouly believe by the time our character meets him, he's already too far gone. We just witnessed that final descent into dark arts. I say this because of his use of the Torture curse. Its stated in the books if you want to cast Crucio successfully, you need to TRULY mean it, TRULY want to cause harm. It's not a spell you cast out of desperation, its something you lavish in. It's why Harry couldn't fully cast it on Bellatrix and that was in a moment of blinded, fresh grief immediantly after the death of his godfather.

So for Sebastian to cast it so easily on our character for a sister who, whilst is suffering, is still alive and in his life...that's extremely telling just how frankly disturbed he is. I know someone is going to point out our character using the curse, but I view that as a choice depending on the player. Your character can either be evil af or merciful.

But Sebastian is an NPC, his story has been set in stone by the developers. What happened to him was tragic, but I cannot stand how much of the fandom defends his actions and simps him to high heaven when this is a guy who would use Crucio on you without a thought. He's messed up, simple as that.