Sacrifice a creature: Falkenrath Torturer gains flying until end of turn. If the sacrificed creature was a Human, put a +1/+1 counter on Falkenrath Torturer.
The ability checks whether the sacrificed creature as it last existed on the battlefield was a Human. (It doesn’t matter what its creature types are in the graveyard.)
You can sacrifice Falkenrath Torturer itself to activate its ability.