Somebody must be at fault and somebody must be blamed.

You left out the key word: "else".

Somebody ELSE must be at fault and somebody ELSE must be blamed.

When that young man said the woman asked him, "Well, it's their fault, isn't it?" and his answer in the negative prompted her to follow up with, "Then whose fault is it?" she wanted to hear that it was not her own fault, and certainly couldn't even accept "It's no one's fault, it was just an accident." She only wanted to hear that someone other than herself would have to pay for her dental work.