Sanhedrin 78 - Group Murder, Partial Recovery
If ten people beat someone with ten clubs and he died, whether they hit him all at the same time or one after the other, they are not liable to execution.
If they hit him one after the other, the last nine are not liable because they struck a man who would have died anyway, and the first assailant is not liable because there is a remote possibility that the man would have recovered had the others not continue beating him. If they hit him simultaneously, each one can plead that the victim might have lived if not for the blows of others. Rabbi Yehuda ben Beteira makes the last one liable. Both derive their rulings from "... if a man strikes all human life ..."
If someone struck his fellow, and the judges assumed that he would die, but he recovered somewhat, and then deteriorated and died, the attacker is liable. Rabbi Nechemiah makes him not liable, since there are grounds for asserting that the victim did not die a result of the attacker's blow.
Art: Joost Cornelisz - Droochsloot - Brawling Peasants