The Wardens do need someone to keep them honest. That someone does need to keep their eye on innocent lives. That said, that someone also needs to bear in mind that the number of murders acceptable to achieve a goal goes up as the goal gets to be a sufficiently great good. (Assuming that the murders are necessary, of course.) And the greater good in mind in this case is the world surviving.
Slavery is when you're forced to work. The mages aren't forced to work. They're forced to stay in one place, and some of them pitch in to keep that place working. Others find work someplace outside the Tower. Uldred for example was not known to be teaching even if he almost certainly was teaching (in a below board, corruptive capacity.) If he could be forced to work, he would probably have been stuck teaching in an above-board position since he was one of the ones thought to be loyal and known to be effective as a mage. Anyway, given that there doesn't seem to be any forced labor, you'd be more accurate if you called it a concentration camp... which, despite the connotations attached due to the legitimately unnecessary and destructive ones the Nazis set up in WWII*, are still a thing we have. People who are sick with something nasty are forced into areas where they can be kept away from people they can accidentally hurt, not as punishment but as prevention; they are concentrated in a camp for public safety reasons.
*The US set up concentration camps, and I think they were called that at the time, but in my experience people usually call them internment camps, probably in order to downplay the similarities between the two systems. Anyway, "internment camps" is another thing they could be called instead of slave camps that would be far more accurate. The US were about as unnecessary as Germany's, since there was no indication that any of the interned prisoners wanted to harm the US, or would be any good at it if they did, or that they could accidentally do serious harm to us like a mage who goes abomination can.
And yet the bulk of the mages never use blood magic and do not go, to quote Sera, all demony. Yet they are imprisoned and forced to do whatever the Templars want. They cannot leave, and in the case of Gwynn, cannot raise a family of their own even in the tower. It may not literally be slavery, but it is close enough that I really can't see much of a difference. If one imprisons one for the potential to cause harm, then eventually all people will be imprisoned because potentially anyone can cause harm. As to being forced to work, what is it if one cannot leave the tower or have a family but must fight in the wars to keep people free - except them of course - if not slavery? I fail to see much of a difference. If you refuse to go to a tower and are discovered, according to DA canon, you and those who harbor you, assuming their are some, are killed. Rather like being a Warden. Drink or die. Serve or die. After all, the greater good, as determined by those with what we have seen is flawed judgement, demands it. That is my darker world view of course.
As the Herald says, we cannot solve our problems with murder or, for that matter, the imprisonment and enslavement of innocents. Just my opinion of course. There should be a better way, but considering DA is set in an intolerant middle age, one supposes it is not going to happen. That is why I always insure that Viv becomes Divine and Alastair king. I am trying to force change where it is so badly needed.
I do not believe any murder is justifiable greater good or not. There have been five blights. How many innocents were murdered out of hand between them? Bioware seems to ignore this part of the ugly truth. Logically, since the Wardens have committed many crimes over the years, they should be kept in Towers like mages and trotted out for blights?