According to the Mishna (26b) a woman who was held captive by non-Jews and held for ransom is permitted to return to her husband after she is released. In such a case, the assumption is that the captors want to be sure that she will be redeemed (al yedei mamon), and will therefore not abuse her. If they had planned to kill her (al yedei nefashot), however, she is forbidden to her husband, since we fear that she was raped, and may even have submitted willingly to the people who were holding her.
In our Gemara, Levi ben Sisi suggests that an example of al yedei nefashot is ben Donai’s wife. Ben Donai was a well-known bandit and murderer. According to the Gemara in Massekhet Sotah his name was Elazar ben Dinai, and he was also known as Tehina ben Perisha. Ben Dinai is also mentioned in Josephus, who was his contemporary, who writes that for more than 20 years Ben Dinai was head of a band of robbers and murderers in the Galilee, until he was finally tricked into surrendering to the Roman governor and was taken to Rome for trial.
Although there is no clear evidence, from the context it appears that Ben Dinai was not a simple professional criminal, but there was a political angle to his activities. It is likely that his criminal band were partisans who fought against Roman rule in Israel (we find that Josephus refers to a number of the leaders of Bar Kokhba’s great rebellion as “bandits”). This would explain why he was taken to Rome to be tried, rather than being dealt with by the local authorities.
According to Roman law at that time, political criminals who were involved in rebellion would lose all of their property, and all of their possessions would be declared ownerless. Thus we can well understand Levi ben Sisi’s ruling that in such a case such a man’s wife would be perceived as being permitted to all.