This raised point, is a valid one. No slave, in a society wherein slavery is legal, normal etc etc; is going to deny their "master". Just is not going to be done. Why? Simple. The Master lines up 3 or 4 slaves, or more, then makes a demand of any one of them. Pick one, pick the demand. they decline? Execute them on the spot. Guarantee, none of the rest will decline. Look at 15th, 16th, 17th, 18th century history. Slaves simply, do not tell their master "no". Now, they will plot then, they will organize to the extent possible. They will revolt if possible. But, they will not, in a one on one scenario, say no. Even if the "Master"is 12 and the slave is 25 and could easily overpower the "Master". The master has but to yell out, GUARDS!; and the slave is not going to over power them.
This is a common failing I see, in slave games. No doubt, because programming and coding the subtle bits of subterfuge and conniving, would be in fact, gawd awfully difficult. MUCH simpler, to create a "failed command" check for the Master.