Human meat is obtained when a cook butchers humans. Humans have a base Meat Yield of 140, but the actual number depends on the Butchery Efficiency of the butcher, damage of the corpse, and a number of other factors.
Simply butchering a human gives −6 mood for all colonists in the colony, and an additional −6 to the butcherer. The former moodlet does not stack, meaning butchering 100 corpses gives the same colony-wide mood as one (the butcherer's penalty does stack). Psychopaths and pawns with bloodlust ignore either butchering penalty, as do any cannibals.
Most humans despise eating human meat, which is cannibalism. Raw human meat gives a −20 moodlet, and cooking human meat into meals (including in nutrient paste) gives a −15 moodlet. Pawns with the Cannibal trait instead gain +20 for raw meat and +15 for cooked meat. The moodlets for raw and cooked cannibalism stack with each other, either way. Mood effects can also be swayed by a pawn's Ideoligion.
Processing human meat, once it has been butchered, incurs no penalties. Human meat can be made into meals, used for animal feed, processed in biofuel refinery for chemfuel, or put in a biosculpter pod, and nobody will feel bad about it unless it is eaten directly.
Butchering a human is already upseting for a good majority of colonists. So in a colony not filled with cannibals, eating it should only be for absolute emergencies, in extreme biomes like sea ice, or to please a cannibal on verge of mental break. For a cannibal, cooking the meat into a fine meal or carnivore fine meal gives the same net buff (+20) as eating raw, and either meal is more efficient than raw food for nutrition. Alternatively, the massive mood debuff might make human meat situationally useful. It may force a tortured artist into a mental break, or help make ideoligious conversion easier, namely from a Crisis of Belief.
Using human meat for carnivorous animals (whenever raw or kibble) isn't cannibalism, so the animals won't mind. It may be wiser to leave corpses unbutchered, as the butchering itself has a fairly large penalty to both the butcher and the colony. Note that colonists still don't like seeing corpses, so watch out for your haulers.
Ideoligion makes this a different story. With the Cannibalism meme and/or precept, raiders become a completely viable source of both food and positive moodlets. Setting the precept to Required (Ravenous) gives the greatest mood buffs, +6 per meal, but also gives a strong debuff when human meat isn't available. This is also possible in the base game with a colony entirely comprised of the cannibal trait. By adjusting the scenario ("Forced trait"), you can set all starting colonists, or every pawn in the game, to be cannibals.