Here, Washington had a great opportunity to gradually emancipate some of his slaves without much to-do: Just keep them with him in Philadelphia for over six months, and the law would free them for him. He needn't even have lost their labor: would a free black in 1790 have had many better options than working for the president of the United States? And I bet they would have worked pretty cheaply, as well: they surely would have felt much gratitude to Washington for adopting this tack.
Instead, he carefully contrived to keep his personal, household staff (all of whom he must have known well) in bondage by rotating then between Pennsylvania and Virginia. I'm generally inclined to not judge figures from the past by modern standards, but obviously the idea that slavery is wrong was well in the air at that time: witness Pennsylvania's law!
For men like Washington, Jefferson, and Madison, I think slavery was akin to opium addiction: they knew what they were doing was wrong, they just couldn't face the withdrawal symptoms.