A very popular custom this year is Little red riding hood:

You have to wonder, when women dress up in that costume what are they playing? They seem to be saying, come force me now.
I understand the slutty costumes, women like to be slutty in a way that is socially acceptable. Many would want to wear such clothes daily if socially accepted. They dress up as slutty nurses, slutty cops, or many seem to have chosen the basic slutty with no other connotation.
I just think these dress-ups let them show off a part of their personality they can't otherwise show. If it's already there, it lets them accentuates it. If it's not there, maybe they hope to create it by first acting that way. That's how we learn to walk, we act like we're walking and then we can walk.