The way I see it, there's a single question here that needs to be answered: Are slaves citizens?
slaves are still treated as citizens, but just as a lower class.
In the mentioned text you just said they're treated as citizens. You never said the are citizens.
The way I see it, the word "citizen" implies that you are a member of that society. As such you have rights and duties that are probably described in the post-apocalyptic version of a constitution.
It's up to the content of this constitution to state what can and can't be done to a citizen. I'm think it's safe to assume that this society of yours is not one that would openly allow it's members to slaughter and rape one another.
So if the slaves are citizens, damage to their integrity by another citizen should be punished accordingly.
I think the answer for this dilemma lies in the requisites for becoming a citizen.
The loophole you're looking for might just be here. If this is a community, I'm certain that one of the duties of the citizen is the periodical payment of taxes. Slaves pay no taxes 'cause they have no money - and that is against the rules. Therefore, he can't be considered a citizen. Wouldn't be fair to everyone who works to pay taxes, right?
Rich people use the cracks in the written laws to enslave others. If you lose the status of "citizen" you're royally fucked. There's nothing stopping anyone from writing your name in a binding slave contrac. Then, the value of your life starts being the exact value of how useful you are to someone else. In the moment you stop being profitable you'll probably be sold (to a maniac or to an even more desperate owner, since he's buying used slaves) or be disposed of.
If the concept that "you are not a person. You have no rights. I can do whatever I want with you" is hammered enough times in the minds of people for a sufficient amount of time, it starts to seem normal.
That's the point where the raping and killing of a young slave would be perceived as an annoyance only to the poor owner who lost a perfectly fit worker and will seek legal action against the perpetrator of the crime, as means to guarantee compensation for his loss.