I don't think the justification on killing relies so much on being enslaved per se, but just whether you are desparate and miserable - and that, to the best of your knowledge (which is the best that we ever know anything), you know killing the person is the only way to alleviate your suffering.

The Haitians were undoubtedly miserable and desperate while enslaved, and, from their perspective, it probably did seem like killing all white people was the only way out. Nowadays, I think we have the perspective to know that white skinned baby isn't predisposed towards cruelty any more than any other babies, but that probably wasn't obvious to the Haitians.

As for baby a Hitler, Leopold, Stalin, or Mao, I think it's a question on whether you think you could intervene in another way. I suspect that if they had the right parenting, therapy, or counseling at the right age, that they would have turned out better, but that's the hard question. You basically just have to do what you think has the best probability of limiting the most suffering.