I don't really understand the idea of, "the subject must be treated with care." I'm not criticizing the statement per se because I'm not really sure at the point being driven at, but I certainly don't understand the underlying implication.
To me if evil is to be evil,
then it must be real evil, not cartoon evil. Or clean evil, or sanitized evil, or whatever the term might be. Otherwise it is not really evil. It is merely a pretense and a facade, without much force, and it does not present much in the way of being a real threat or danger. (And if evil is not a real threat or danger, then it is toothless, and clawless.)
And if evil, real evil does not exist, be that on the individual level (crime, terrorism), the monstrous level (some monsters are definitely evil and destructive), or the social/cultural level (group injustice, repression, slavery, human sacrifice, etc) then what is a Hero to agitate against, and what is he to fight against, and just as importantly,
what does he fight for?
If slavery exists in my setting, and it does, though it has been mostly suppressed by the Empire as an idea and practice, then to me it must exist as it really did, with a wide variety of expressions, most all of which are repressive to the individual to some degree or another. (If nothing more it would restrict your freedom of movement and individual association - which to an adventuring Hero would be practices anathema to their very nature I suspect).
But the wider idea to me seems to be that you present Evil as evil is, if it is to be real evil.
Then let the players decide which evils they think it most important for them to address and fix (if possible) depending upon their own standards of priorities. (That is to say, do they consider this bloodthirsty human trafficking and murder gang the greatest immediate evil because they are organized and ruthless, the village slaughtering yet basically mindless and brutal Ogre the most present danger, or the practice of enslaving and abusing and possibly sacrificing others based upon race, or ethnicity, or class, or some other measure elf what constitutes a slave the underlying threat which makes it possible for the Ogre to kill un-opposed, and the gang to operate with cover and unmolested?)
There are advantages to eradicating each kind of evil, just as there are disadvantages to eradicating each in a particular order, but to me being a Hero is all about eradicating evil. The difficult part really comes in in being forced to choose what evil you address given your particular situation, and why, and which you are willing to accommodate yourself to, at least for the moment, in order to achieve what you consider some higher or best end.
But to me you can't really make those kinds of choices and decisions unless you know what you are really facing, and you only know what you are really facing if you understand your enemy as the enemy really is. And if you don't know the way the enemy really behaves, and you don't know the true aims and methods of some kind of evil, then you really don't know that evil for what it truly is, and therefore cannot know how, or even why, it should be opposed. In other words you cannot be as Good as you need to be, unless you first realize how Bad a particular type of evil actually is in nature and behavior.
(Now I can see making an exception for a children's type game setting, where children have to be introduced to the concept of evil gradually and gently, so as not to shock them as to the way things can unfortunately sometimes really work.)
Other than a few specific exceptions though, as far as I'm concerned, you present Evil as evil. Butchery, and all.
Then let the players decide what they want to do about it. And why.
I'm using a Greek-based setting, so the idea of slaves as being much more than gladiators, sacrificial victims, or expendable hard labourers makes perfect sense...
Unless they're Helots. And I'm just kidding of course, I read the last part of your statement as well.
