For a handful of nuts a non-rev, who is otherwise entitled to sit where she is, should go to the back of the plane? Or not even get on the plane? Mentioning an serious allergy is self-centered? I don't think so.

OP was fine. I might be frustrated too and I have a food allergy. But people don't all communicate the same way. They may stumble over their words. And sometimes they just have a bad day. A little grace goes a long way.