You're right, the "Castro freed slaves" meme is not a good one. It's wrong on its face (people will do the same basic search you did and see 1886), and no matter how bad working conditions were in 1959, they were far different from chattel slavery. Your boss couldn't just kill you for amusement or sell your wife and children to some plantation on the other side of the country.

Besides, you can say "Batista was a murderous dictator and even the U.S. agreed a revolution was necessary:"

The third, and perhaps most disastrous of our failures, was the decision to give stature and support to one of the most bloody and repressive dictatorships in the long history of Latin American repression. Fulgencio Batista murdered 20,000 Cubans in seven years - a greater proportion of the Cuban population than the proportion of Americans who died in both World Wars, and he turned Democratic Cuba into a complete police state - destroying every individual liberty.

Yet our aid to his regime, and the ineptness of our policies, enabled Batista to invoke the name of the United States in support of his reign of terror.

Administration spokesmen publicly praised Batista - hailed him as a staunch ally and a good friend - at a time when Batista was murdering thousands, destroying the last vestiges of freedom, and stealing hundreds of millions of dollars from the Cuban people, and we failed to press for free elections.

This is something that gets more true when you look into it, not less.