In the five years I observed vice in Maquis Park, there was only one fatality for a prostitute who was managed by a pimp. In contrast, thirteen self-employed prostitutes died during the night hours at work. Of these, ten died at the hands of either an abusive john, a spouse or partner jealous of their work, or a pimp trying to clear them away from a spot: the other three died of drug overdoses, although they too may have been dealing with some type of harassment.
Sex workers with pimps can earn more money, and their work is more steady.
Shades of Walter Block. From this data set it does seem that pimps beat their prostitutes a lot, but that otherwise there are more beatings. That is from Sudhir Alladi Venkatesh, Off the Books: The Underground Economy of the Urban Poor. Here is my introductory post on the book.