Many references to Twain's opposition to slavery in any form can be found
throughout his work. You might want to look at the entry under "Slavery" in
the Mark Twain Encyclopedia edited by Jim Wilson. Turn to the Autobiography
of
Twain arranged & edited by Neider, not a favorite of scholars for textual
reason
s, but handy enough to extract the kind of stuff you're looking for. LOOK AT
the end of Chapter 3. And this from Chapter 8:
        As I have said, we lived in a slaveholding community;indeed when
        slavery perished, my mother had been in daily touch with it for
        sixty years. Yet, kind-hearted and compassionate as she was, I think
        she was not conscious that slavery was a bald, grotesque, and
        unwarrantable usurpation.

That's a beginning. Look at other works by and about Twain and try using the
index to extract some indications of the full range of his loathing of the
practice.
ACR