A fool who was obligated by the Torah according to a rabbi

Does the Rabbi have an explanation in the Rabbinical system that the liability in a pit is on the vanity and not on the blow? It seems that according to his system, a woman who broke her leg in a pit is exempt because it is clear to us that the damage was caused by the blow. Moreover, it is not at all clear what it means to be harmed by vanity, for even if there is vanity in the pit, it will probably kill, and if it does not kill, it will probably do nothing.