It would probably be a good idea to not have the slaves rebel when there is no combat going on. We don’t want to force the player to consider such mundane questions as “what is fastest route from A to B”, and so on – outside of combat, exploration should be encouraged. Perhaps it would be nice to have the chance that a slave rebels on its turn be equal to the current tension? At 5 tension, the slave has a 5% chance of rebelling, and so on. That is pretty intuitive, easy to explain, and it also makes it clear to the player why it can only happen in combat. Plus, it makes tension even more tense than usual.
The player should certainly be able to attack his slaves. I agree with the idea that you get a bonus on your first attack, after which they immediately turn hostile. Any slave which has been betrayed in this way cannot be enslaved again.
Perhaps the slave, when it rebels normally because of tension, doesn’t turn hostile immediately – you see a message saying something like “'s soul is slipping from your grasp”, and it only really turns on you on its next turn. This would allow the player a last opportunity to backstab her slave, perhaps killing it and gaining its power in the process.
(By the way, I need to redesign the factions system to make all of this possible; or rather, I need to clean up the mess I made of it when I decided that factions and monster types were equivalent. For instance, undead are a faction, and when the player turns undead he actually becomes of a different faction, which is why Fafhrd and Mouser will attack you when this happens. Which was intended, but of course it wouldn’t work well with this new slave mechanic, and in fact it was a terrible design decision in the first place, because it rusn to conceptually different things together. I have no idea why I decided to do it that way.)
Perhaps it should get a number of hit points bak based on the player’s body score? 5 + body/2, something like that. (Your slaves will always be level 0, level 1 or level 2 creatures, so getting getting 7-15 health back would not be inconsiderable.)
Yes, it always works this way with any person that is killed in any way: the player absorbs the soul. (Well, if the abyss of the soul isn’t present.) Fafhrd and Mouser, and undead, cannot cheat you out of souls.
Nothing ever happens in rooms where the player is not, so this scenario could never happen.
Ok, that is not exactly true: there is one creature which can kill other creatures when you are not present. You can exploit this for a non-standard victory. Spoilers below. But it is not important for the current discussion.
The Nameless Horror, which you can sometimes release by digging into the Eternal Prison, will follow you through the dungeon, killing anyone it meets. Including Malygris, which will win you the game… though with a rather pyrrhic victory message.