Peleus was not quite as experienced with their son as she was. Maybe he was coming into this now as an outsider. That's all he'd been for a long time, a title, a name, a figment of Achilles' own head and not the figure he should have been. He'd betrayed his own son a long time again.
That was where he'd failed him.
This was his shining reward, a shiner and a busted nose. More than that, his son's hate. He took a deep breath himself, matching that concerned sigh that she had given. Achilles had hardened himself, he'd become very good at guarding himself. Now that he'd let that damn break and feel something it was too much for him to take. All he knew to do was lash out. "He's punishing us both...." for what? Maybe Peleus had expected Achilles to be a little more accepting of them than he had been.