On first watch "Slave" struck me as a very powerful episode and one of the episodes of Season 6 that hit me hard emotionally. Which took some doing as there are quite a few episodes in the season that stirred emotion in me. It has one of those subjects that makes one feel a lot of anger and sadness internally and externally, emotions that is felt naturally rather than feeling forced to feel those things. Something not unusual for the 'Law and Order' franchise overall before and since.
My feelings on "Slave" are exactly the same on rewatch and perhaps slightly stronger, due to getting a little more out of the story a few years on. It is not quite one of the very best episodes of Season 6 but it is very close to being up there. There is one aspect that have done with having more dimension, but the execution of the difficult subject matter is beautifully handled and the two main guest star performances leave a huge positive impression.
Almost everything in "Slave" works beautifully. The episode is slickly photographed throughout, a perfect match for the gritty tone, and New York looks both striking and atmosphere-filled. The music is only used when necessary and when it is used it does stick in the mind and not done so ham-handedly. Both the main theme and opening voice over are memorable. It is directed with a confident and sympathetic edge. The script is intelligent and concise.
Nothing in the script feels extraneous or too flowery, there is no trouble understanding what is being said and there is not a sense of just one side being taken. "Slave" has a powerful story that has scary tension and also heart-wrenching emotional impact, have always been left with goosebumps on every watch. The character interaction between the regulars, especially in the legal scenes (though the chemistry between Briscoe and Curtis has come on a long way since the beginning of the season), is reliably strong.
"Slave's" supporting characters are very interesting, especially Cassie who leaves me feel both hate and pity for. It is also incredibly hard to not feel for Nacho for what he has to endure. Karen Young does resentment and vulnerable so well, it was both unnerving and affecting watching her. All the regulars are great typically. The performance that captivates most though is a heart-rending Adam Zolotin.
Did feel however that for a character meant to be despicable, Morales could have done with being written with more menace and less stereotypically. Augustin's performance is to me a little too broad.
In summary, great and very close to being one of the season's very best. 9/10