Is it...

• the red eyes, when it is out, he can't control himself and is unstable?
• his friends? Kurapika cannot have Gon and Killua killed. The hostage was not equal because Kuroro didn't mind being killed.
• Chain Jail cannot attack Non-Ryodan members? But how did Kuroro find this out?
  • Pakunoda knows this
  • She imparted this to the rest before she died