Why did they want to butcher Louison if....

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I think they were willing to take whatever they could get--they didn't like killing their own--and they kept mentioning he was skinny because they were lamenting that their latest catch couldn't have been a bit meatier.

~^~Help! Babies ate my dingo!~^~

You'll notice when the buthcer first meets louison he turns him down immediately because of his size. But I believe he figured he could exploit the skinny man for some free labour because as all the tenents said, the building was in terrible condition. Later in the film when Louison is still very much alive the butcher makes an offhand remark to the postman about the newspaper advertisement which led me to believe that the ad was still running in the paper even as Louison was working in the building. At a point I believe they just became desperate. The butcher even went so far as to kill a useless old woman who "would not even make a good paté." Everyone else in the complex was in on the agreement so in essence Louison had simply drawn the short straw.