Two girls are having kinky sex and switching roles, and keep egging each other on / daring each other to escalate. Like "oh you hit me 5 times, I'll hit you 10 times" "oh you can take 15 times, I can take 20 times" etc. Eventually (maybe over a long time, maybe during the same session) through multiple kinks (spanking to knifeplay for example) they escalate to mutilation/ death and both end up dying / killing themselves / each other, all while having good competitive fun and with 0 regrets.