Responses
From what I've read at least from that arch the Sin doesn't seem to corelate to whether they do a crime or not but more to whether they feel like the thing they've done is a Sin or not. The pill girl only develop her Sin after she heard the situation surrounding her friend suicide even tho bluntly speaking it has nothing to do with her.
So with that pov we could take the bullies doesn't consider what they do as a crime/Sin and so they don't develop a Sin
This story is a bit too inconsistent with sins. So the paper son retaliate against the father and automatically get a sin, even before killing him. The father got none after years of abuse. The high school guitarist protected her friend and got a sin but the bullies have no sins, despite constantly beating the pill girl up? Also let's not even talk about how that pill girl got her sin. So being suicidal/depressed is a sin now? Where is this author even going with sins being being created from retaliation but not from the first aggression?