I think he's jealous.

“Cassandra Clare“Malachi scowled. "I don't remember the Clave inviting you into …”

“Will had shrugged once, helplessly. He had almost wished Jem would be angry wit…”

“She gritted her teeth. She was here with Sebastian, on her way to see a powerfu…”

“Jace likes to pretend that everyone isn’t talking about him, even when he knows…”