Damian one day got into a fight with his father. He was being blamed for because of something he didn't do. Batman lashed out at him. The final straw was when Damian got smacked. Damian ran to his room and ran away in the cold dead of night. He eventually collapsed. Yes he was an assassin but he was also just a kid. There was certain things even he couldn't handle. That's when Jason found him and took him in. After he found out what happened that when all the drama started.