I like to think them not as slaves, but as demonic prisoners with jobs that my warlock has contractually obligated them to for an indeterminate amount of time and no guarantee of a safe and happy work environment.
Nah, those demons aren’t slaves.
Warlocks think they are in charge but the demons are just playing along, waiting for the right moment of weakness to drag you off into the Maw or where ever WoW demons come from.
Do not worry Izoth. This character will be unenslaving many demons in zones with a dense population of people who are low level when I reach max level.