I read in on a book on Haiti called "Written in Blood: The Story of the Haitian People 1492-1995". In that book it gives the brutal details of how french troops used to try beat the rebels.
But could you give it at least the part where it mention this, and if possible from which this indication came from?
As I said, it's no part of the secret instruction (they go as far than enforcing slavery re-establishment, so I don't think they would have made manners about mass killing), or the official ones of course.
To be honest, I found this nowhere else, so I hope you'd understand me there : it must have been seen or accounted somewhere in contemporaries documents, but which ones?
On your second point if they try not to genocide the blacks they will never control the island. It will become mini afghanistan. The blacks are free and know what freedom is. It will be dificult to just reinslave them.
I agree, it's why I think they would go first for their leadership, that was mainly metissed or free blacks (as in, free before the Revolution), rather than former slaves.
Without these (and that would be clearly a brutal slaughter), re-establishment of slavery would be certainly more easy, if they don't eventually try some half-measured remplacement, as a compensated slavery in facts.
So if they do not go through ethnic cleansing they will never control the island. I doubt Nappy and pals would have patience with this.
Thing is, they can't do that without loosing the alliance, even if it was a plan in first place (it's not like the brief reconquest launched an immediate genocide).
Killing all slaves, when you can't bring other slaves would be insanely stupid. It would defeat the purpose of taking back the island, and make it more costly with absolutly 0 benefit.
Giving Napoleon personality, it's more probably that if they are in such situation they can't do shit about it safe genocide, they would just call it a quit and racket independence as IOTL.