You're not the only one: I never go to slavery as a civic, because I never whip. Game-wise, I may be losing out, but that's not the important thing: we play this game for fun, and for me slavery and killing people, even virtual people, is no fun.
It's not a moral thing: I paid good food for those people; I can do better than turning them into soylent green. Keep them around and keep them happy and they will repay me with production and commerce beyond my wildest dreams!
Maybe I'm just an old softie, but my empires are, for the most part places I would like to live in. Unless you're a discontented, recently-conquered city, and then I'll let you starve, you ungrateful little bastages!