[From the perspective of someone trying to abolish slavery] kind slavers are the worst because they make the situation just tolerable enough that people aren’t motivated to change the system.
[From the perspective of an individual slave] kind slavers are the best because they suffer the least.
Kind slavers force a system settle on a local minimum of suffering rather than a global one.
Who says kind slavers weren't also helping to change the system? It's a pretty big assumption that all kind slavers were only being kind to keep their slaves from being uppity. Re: George Washington
Depends on the meaning of "own". Quoting from https://www.mountvernon.org/george-washington/slavery/martha...
> After George and Martha Washington married in 1759, she took many of these people with her to Mount Vernon. As property of the Custis estate, they could not legally be freed or sold, and were inherited by the Custis heirs upon Martha Washington’s death.
In this case, Martha Washington owned the rights to the labor of the people involved until she died, but did not own the people themselves; the Custis estate did.
(Now what she would have done if she did own them outright is a separate issue, of course. Nothing suggests she would have freed them.)
1) The ones owned by George Washington himself. He did not free these while alive; he did have a will that said that once Martha died they were free. That is, he did not free them until he (1) made use of their labor himself and (2) ensured that what he viewed as his financial obligations to Martha upon his death were satisfied. Obviously this is not a hard-line abolitionist stance, and I do wonder what he would have done if he had living children when he died. As it was, he just had living step-grandchildren (kids of Martha's son from her previous marriage; the son had died earlier and George and Martha had raised his kids).
2) The ones owned by the Custis estate. Those could not be freed unilaterally by either George or Martha. They were inherited by the above-mentioned step-grandchildren when Martha died.
What would happen for people in set 2 if they were in Philadelphia for more than a year is a legal question I don't have an answer for... The most plausible way to reconcile the various legal bits I can think of is that they would be free but then Martha (or George? family property law in 1700s Virginia is not my strong suit) would owe restitution to the Custis estate for the lost value or something like that?
The rules were considerably more complicated for Pennsylvania residents; the law basically grandfathered in existing slaves owned by Pennsylvanians and provided for a decades-long slow abolition as the children of existing slaves reached the age of 28.
It's interesting that the law exempted members of congress (and representatives of other countries, possibly for similar reasons) but not the president.