I would speculate that it is a simple matter of your roommate not fully understanding what a D/s relationship or the meaning of "Dom"/"sub" is.

I am hesitant to serve anyone who isn't caring and kind on a regular basis... but also cruel and evil (as defined in the fun sense of the word) when the time is right as well. And no matter if they are caring/kind or cruel/evil they can still be Dominate over me, based on several factors.

So... long answer shortened. I would say perhaps your room mate needs a few lessons on what BDSM truly means beyond a few porn shots or something that don't show the full realm of a relationship.