How does tom's plan go awry?

Tom finds the Judge downstairs, asleep on the sofa. The old man's tin cashbox stands nearby, closed. Close to this is a pile of bank notes. As Tom seizes his prize, the Judge grabs him and starts calling for help. Tom plunges the knife into his uncle, and he is free. He flings the knife onto the ground and runs upstairs to his room, just as the twins are entering the house and standing over the body.