It's a little unusual for somebody to attack several prostitutes and not sexually assault them. Often, serial attackers of women are profiled as gaining pleasure from their dominance over their victims or as using them as an outlet for rage of some kind. This usually includes sexually assaulting or otherwise brutalizing them.

As far as serial killings go, I think that it is actually quite unusual for 5 prostitutes to turn up dead without a scratch on them and without having been sexually assaulted.