The only effects Slavery should have on dissent is from the civic itself (high dissent), and dissent from the unhappiness generated by whipping. I'm guessing that dissent is lower in the city because you're killing most of your unhappy citizens. Check the dissent advisor - which column is showing lower dissent when you whip? Overall dissent is pretty low on Warlord difficulty, wouldn't surprise me that it's possible to have games without civil wars there with the current settings. I'm currently working on ways to make breakaway civilizations less 'diplomatically isolated'. Sometimes the AI is going to be stupid no matter what though.