There's all sorts of violence. A slap can express as many things as a kiss, maybe more. And no doubt there are a lot of men who call themselves doms who are basically sadists and misogynists.
Why is rage so close to passion? Why is anger and threat often so sexually exciting? I've got a lot of theories about BDSM. On the one hand there's the obvious one that says that BDSM is an exaggeration of the usual sexual stereotypes of the aggressive male and the passive female. It's almost as if people who are into D/s have a need to act out these roles in the extreme. The man wants to dominate and control, the woman wants to be taken and controlled.
I have another theories too. Theories are easy. Whether they're true or not is another matter.
---dr.M.