blaming yourself for a guy that killed everyone he could just because, that ain't it chief
Yeah but that is survivor's guilt for ya, it doesn't have to make sense it just is, and he has been living with it all this time.
He has been trying to figure out how he could have saved her, basically living in that day forever, it is really sad and depressing because he did as much as he could as a child, he literally stood in front of that man thinking he was going to die to give his sister enough time to run away.
But the fact that in spite of all he did she still died it hurts more than if she had just died while they were both running away, because he took an active part in trying to save her, he believes it was him that failed and that wasn't good enough