True cannibalism is impossible for non-adventurers in Dwarf Fortress. You can easily set an entity's ethics for [EAT_SAPIENT_OTHER] and [EAT_SAPIENT_KILL] (it might only need to be one, but just go with both to be sure, no real downside) to ACCEPTABLE will allow them to butcher intelligent creatures other than members of their own entity, which is probably what you wanted out of cannibalism anyway. You have to mess around with it a bit as you need to get the corpses near the butcher's shop to get them butchered just like if you had your military hunt down animals, but that's not too difficult. Cannibalism in adventurer mod used to be difficult, you supposedly had to be starving before you could do it, though despite trying I never managed it, but now you can butcher intelligent creatures whenever you want.
However, in 0.42 Toady accidentally messed something up and made it so you can't use items from intelligent creatures under any circumstances. Toady doesn't even know what happened that caused this. In adventurer mode you can butcher the creature, but then all you can do is lick the meat, or carry the skull as a trophy or whatever, no actual cannibalism. There is a trick some people have done where you can make an interaction to make a creature unintelligent before killing them, but this is unlikely to work well in fortress mode.
Basically if you want cannibals it has to be in 0.34 or 0.40 because they were broken in 0.42.