One view you can take, which seems defensible, is that he grew to agree with the anti-slavery view late in life, yet as a calculated measure to hold the country together when half the economy was dependent on slavery, did nothing about it, and also was too weak to give up his own station in life which was similarly dependent on slavery. Yet he freed his slaves in his will, both for their own sake and in the (ultimately vain) hope that he would inspire others to do the same.
The other view is that he never actually believed it, or he would have made freeing human beings a political priority. He was embarrassed into freeing his slaves after Martha's death, but he fundamentally thought that it was more important for himself and Martha to live their last years in comfort than for his slaves to live in freedom, and that the negative peace of the new country holding together was preferable to the positive peace of meaningful liberty. And that therefore the "good" he did for this country was to set us up for the Civil War and for many more decades of viewing certain people as not fully deserving of human rights.
And relevantly to RMS and the free software movement - if Washington hadn't been there, if instead William Cushing or Gouverneur Morris had been in a similar role, what would they have done? Or even if Washington were still there but he did not use his leadership position to say, "I don't like slavery, but we have to keep it for now," what could Cushing or Morris have accomplished? Washington presided over the convention where Morris made his ultimately ineffective argument.
Washington wasn't the only founding father, nor the only skilled military leader among the revolutionary forces. In a world without him, would the US still have won and would it have been better set up to fight slavery? (Would the revolutionary forces have allowed black soldiers in earlier, and moreover had more morale among the black soldiers, thereby leading to an earlier victory?) If such a scenario is plausible, then the good he did didn't outweigh his wrongs.
I'm just certain sure his slaves felt the same way. Especially Ona Judge.
