Suppose there are two people: A & B. They are mean to each other. A treats B with a lack of respect which causes B to yell at A which causes A to treat B with a lack of respect, etc. It isn't clear when this began, whether it began with A's mistreatment or B's yelling.
Is the answer to the question "Who started it?" important or revealing, when it comes to issues of moral responsibility? If both of them are sustaining it, doesn't that make them both equally morally responsible (all other things being equal)? Or does it matter who started it?