Yes, I believe in a Creator. I arrived at this belief thru reason coupled with intuition, and, also thru reason & intuition, I conclude morality is definitive (not subject to interpretation) and objective (applicable to all, everywhere, every-when; independent of what anyone likes or dislikes about it)
No, the big blank book of deism doesn't say slavery is wrong.
I might, for all you know, think enslavin' certain folks is a grand idea, but I don't becuz I know it's wrong to use persons as property.
But, enough of that: i'm not here to promote or defend deism. What's on the table: does atheism offer a moral grounding? It doesn't (in the same way materialism, determnism, scientism, etc. do not).
The atheist (materialist, determinist, or science worshiper) may dislike slavery, but can't oppose it on any grounds other than distaste.