Although Ian tried to comfort her, Aria was still crying hysterically. She was scared and the fact that she had killed a person was etched in her brain, heart, and soul. It was eating her alive and she was losing it quite badly. Killing a person was nothing to Ian, not when the man was a traitor. But that was not the case for Aria and he knew about it. Aria was an innocent soul and never would she thinking of hurting anyone.
Ian could understand how she was feeling as he too had felt the same when he had first killed a person who had tried to harm his mentor, Theo when he was still young. But it was his mentor who had told him that it was nothing wrong in killing a person who was trash and an unwanted baggage to the earth. The man whom he had killed was a serial killer and a rapist. So, Theo did not scold Ian for what he had done. Instead, he had even praised him for his bravery and his skills.