I am speaking strictly morally (or ethically if you prefer). Jefferson obviously had the legal right to own slaves. The fact that the laws themselves were unjust is my point.
They are unjust by modern morality, morality changes over time, it is not absolute.
It's more like morality is unchanged (Justify something other than: 'do unto others', without falling back on an obviously illogical subjective value system) but society's dirty excuses for immorality change.