Some people say it as an exclamation of exasperation and not meant as a personal insult.
You’d have to hear tone to gauge it. If she included a 2nd person pronoun then it’s a direct insult.
Direct insult, yes. But to the “son”, not to the mother.
It’s saying “you’re the son of a dog”.
Exactly. As she stood up to come to the door she hit her foot on the table. Or she broke a nail. Or spilled the coffee. It could be a million things that had nothing to do with the guy. Just a culture gap that spun out of control.