Cows and pigs are slaughter rather humanely with a pith gun, chickens get their heads cuts off while place upside down on a coveryer belt. I would advocate just filling a room up with nitrogen but that would be too expensive.
What hell is how cows, pigs and chickens are raised in factory farms: life in tiny cages or crammed with many others or both, bred to be so meaty they can barely move even if they had the space. We once had a factory chicken at our farm once: it was an ugly thing, huge and fat as hell and its feathers were all deformed, it could barely walk and it was completely blown away by the concept of an outdoors it took it some time to figure out that it needed to go back in and roust with the others ever night (either that or it was stupid as hell, probably the later), and it did not live very long, apparently so many breeding generations of having very short lives (six weeks from egg hatching to head chopping!) that its life span was shorter then regular chickens and it aged much faster.
I guess if the breeding continues we will have breeds that lack brains and are just hunks of meat that we pump food into and pump excrement out off, and maybe shock with electrodes to give the muscles a firmness.