Imagine Peter standing up for you against your rude ex:
You told Peter to be nice, to be casual, but he just couldn’t help himself. You’d brought him to pack up your stuff, moving it to your new place far away from them. They’d been civil up until this point, when they spoke so crudely of you to Peter in passing conversation, as if they were talking about the weather. He couldn’t believe it. Peter was sure you hadn’t heard from the other room, grateful, but that didn’t mean they were going to get away with it. When you were finished, taking the last of your boxes out, he took the chance to flip him off one last time, telling them how it is. That they were stupid for ever treating you badly, and that they’d never find anyone as good as you for as long as they lived. You were going to say something, wanting to leave the relationship on good terms, but there must have been something said to set him off. You trusted Peter, knowing he wouldn’t have snapped like that without a reason.