At the heart of the problem with accusations of the immorality of slavery in the Bible is the differing definitions of slavery.
I’ve provided the modern definitions of slavery and I’ve provided the definitions of ebed and doulos. Neither ebed nor doulos match what is the modern definition of slavery. So, to make any equivalence is to commit the equivocation fallacy.
In logic, equivocation (“calling two different things by the same name”) is an informal fallacy resulting from the use of a particular word/expression in multiple senses within an argument.
It is a type of ambiguity that stems from a phrase having two or more distinct meanings, not from the grammar or structure of the sentence.
Even more, the word “slave” is never mentioned in the KJV. In modern Bibles, translators have a hard time translating ebed and doulos and cannot find the right English word, sometimes they use servant and sometimes they use slave.
In the modern definition of slavery, the general view is it involves involuntary servitude which is exploitative and abusive. In the Biblical context, ebed and doulos simply involve servitude, without stating whether it is voluntary or not or exploitative or not. In the vast majority of cases, ebed and doulos do not match the modern definition of slavery.
To avoid the fallacy of equivocation, we need to qualify the definition to be more specific to equally compare what is being referenced in the Old Testament. Simply using the words “slave” or “slavery” (without any qualifications) to make any statement about the Bible is equivocating.