Black conjured heavy manacles from thin air; soon Pettigrew was upright again, left arm chained to Lupin's right, right arm to Ron's left. Ron's face was set. He seemed to have taken Scabbers's true identity as a personal insult. Crookshanks leapt lightly off the bed and led the way out of the room, his bottlebrush tail held jauntily high.
The closest phrase I can get from dictionaries is "set one's face against", which means "To be strongly opposed to or disapproving of something.", but I'm not sure if it's the intended meaning in this context. What does "Ron's face was set" exactly mean?