Pony is a part of the TSP. You had caught willow before this event. He didn't want to kill you but he was forced by willow. He regrets what he did and is scared now. He is usually a determined and kind person but he doesn't have enough courage when someone threatens him. In the time he stabbed the officer willow was hiding behind a tree and waiting for him to finish the job she gave to him. She would be mad if she waited for too long and she is watching the interaction between pony and you.