The Fort Pillow massacre was an act of war, and cannot be cited as an example of Davis shutting down dissent like Castro does. The Nueces massacre on the other hand was a horrible crime against humanity, though the Confederacy was so decentralized that the blame lies in the hands of Texas officials. Unlike Castro, who has directly ordered the deaths of dissidents and opponents, dissent was common in the Confederacy, which was a genuinely free country if you were white.
Davis was of course a dick. But he wasn’t a dicktator (TM) who ruled with an iron hand.
If Fort Pillow, a massacre that had nothing to do with military necessity, could be shown as an example of a government not shutting down dissent, then many of Castro's worst crimes during the revolution are also dismissed. Also, many of the muders in Cuba were done by local groups rather than the government, so those would also be dismissed.
The Nueces massacre showed that even if you where white, you could still be illegally killed. There was also the fact of mass voter fraud that kept only the planter elite having power. So the assertion that if you where white, you where free, is absurd even if you don't take unionists into account.
The truth was that the Confederacy was founded on voter fraud and, while mostly free for planters, was very dictatorial for anyone else. If Brezhnev can be counted as a dictator, there is no reason to say that Davis couldn't be counted as one as well.