Under My Thumb is a shitty, shitty song. That's not open to question. At the same time, it's not literally a celebration of rape and murder. FF is talking in very crude terms and his line about "black privilege" is ridiculous, but I get his point when he talks about relativism in this particular context.They're writing in the 60s, Geto Boys in the early 90s, which I think explains the difference in form - intent is much the same. But Brown Sugar, Under My Thumb etc are grimly misogynist, and obvs racist too in the case of the first.