Once on the roof, Crypt pulls the knife from his shoulder, "tch, looks like I'll be spending my check on the medical bill, f***," he swears, heading towards the hospital, he could handle the pain, but it'd be a detriment at work, and his mother would worry, which could cause her condition to worsen. He keeps the knife, rolled up in a book cover in his pack, it was evidence he could use against her. Better to keep her off the streets, and throwing a knife like that would be a oneway ticket to Juvie, or Jail.