There are a lot of things Stiles doesn't talk about. He hides behind whatever nonsense he's been reading on the internet lately and the new research he has, and it works. No one notices the way Stiles never talks about his mom. Or the fact that his father no longer has a job because Stiles took him for granted.
The problem is Stiles feels things too deeply. He internalizes the wrong things and spends all of his time trying to fix things that he believes are his fault. And maybe they are. Maybe if his mom hadn't had to deal with a hyperactive disobedient little boy while struggling with cancer, then she could have fought it better. And his father wouldn't have had to work such long hours to provide for Stiles.
It's definitely his fault that his father was forced out of his job, because Stiles stole a fucking police vehicle and kidnapped a fellow classmate. It's his fault that Scott is stuck between pack dynamics and his girlfriend's hunter family. Cause if Stiles hadn't dragged Scott out of bed that night, then Scott would never have become a werewolf.
If Stiles had stopped fawning over Lydia who had never given him the time of day then maybe Erica wouldn't have felt the need to join Derek's pack. And, god, Derek. Stiles can't claim the blame for Derek's whole family dying in the fire set to his house. Kate Argent had claimed that one proudly. But Stiles stole Derek's last family member. He set Peter hale on fire all over again, and hadn't even done that properly. It was his fault that Derek had to kill his own uncle, the last of the Hale family.
So if, sometimes, when Stiles comes home from seeing more trouble come from the messes he's made and he can't distract himself any longer, Stiles reaches under his mattress and pulls out his blade. And if Scott notices that Stiles smells like fresh blood the next day, he'll just dismiss it as Stiles being clumsy again. Because Stiles needs to pay for his mistakes in someway, and this is the only way he knows.