The thing is that random creatures entirely
should have the capability of being sadistic - just like they have the capability of being anything else. The random nature of personality tags exists for this precise reason. There are a few other things to keep in mind, as well. For instance, the following:
We've got procedural cultures coming soon. What this will mean is that dwarf civs won't necessarily have the same laws and ethics as they do now - when modding, for instance, a user can specify all the ethics they want to keep at a static value, and the game'll procedurally figure out the unspecified ones based on the situations the civ encounters, the traits of its leaders, etc., etc..
By suggesting that sadism should be an "on/off switch", as you put it, you're completely removing an entire facet of that procedural generation. In addition, if you're so worried about how complex the integration of sadism as a personality component would be (I'm sure you know this is not a good community wherein to argue against overse complexity), consider the implications of it
not being a personality component in a fully procedural environment - using the procedural culture example above, there'll have to be a check specifically for whether the civ is sadistic or not and measures in place to stop inappropriate ethics and laws from forming, and so on. Why not just put it under the existing model? Your only counterpoint being...
So what, something like this should wait 5 years until personality traits actually do something?
Or something along those lines. And yes, it should. Mind you, currently, the only thing barely resembling sadism is the ABUSE_BODIES tag, which should be mainly redundant in the next version, due to the aforementioned procedural civs.
There is no "moderate" sadism, although it COULD be suggested that there be different degrees of sadism, but what's the cut off?
Easy enough in theory. Take an existing personality facet:
91 - 100 Is constantly active and energetic.
76 - 90 Is very energetic and active.
61 - 75 Is very active.
60 - 40 n/a
25 - 39 Is relaxed.
10 - 24 Lives life at a leisurely pace.
0 - 9 Can't be bothered with frantic, fast-paced living.
91 - 100 Is obsessed with physically and mentally abusing others.
76 - 90 Is rather amused by the suffering of others.
61 - 75 Is somewhat harsh on its/his/her enemies.
60 - 40 n/a
25 - 39 Is uneasy around cruelty.
10 - 24 Is highly disturbed by the abuse of others.
0 - 9 Is absolutely disgusted by and intolerant of the abuse of others.
It... sorta works.