This was Jace being brave. Simon thought, brave and snarky because he thought Lilith was going to kill him, and that was the way he wanted to go, unafraid and on his feet. Like a warrior. The way Shadowhunters did. His death song would always be this-jokes and snideness and pretend arrogance, and that look in his eyes that said 'I'm better than you'. Simon just hadn't realized it before.