Additionally, it should also be noted that roleplaying could be used as a grooming/normalizing tool to encourage individuals who may not initially be willing to participate in such actions IRL. Everyone likes to think that they have mental boundaries and won't succumb to extreme behavior if encouraged to do so, but social pressure is a hell of a thing; if someone joins a group of practicing sadists, even if they are dedicated to keep that shit in fantasy only, it's very easy for them to get brought around into actively participating as they get emotionally close to people who are actively participating.
This mentality happens a lot in cults and enclosed social groups; both parties maintain respectful boundaries for a while in a "I don't practice that way but they're nice to me so I'll be nice to them" sort of agreement. Eventually, however, as relationships get more intimate you will often see one or both parties adopting the thinking of the other; this can have a positive benefit, if the individual practicing sadism starts adopting the mentality of the individual is against it. However, it's equally likely (probably even moreso) that the individual who is has a moral objection but is willing to "live and let live" can be swayed into becoming an active participant eventually.
Without a solid external structure and a hardline stance against such actions, people will eventually adopt the same mentality through social osmosis.
Edit: Wrong posts quoted somehow.