They are killed in an instant in a rapid grinder and used for animal feed.
There is a large difference between killing animals as swiftly as possible to use as much as possible and knicking off a small bit they need to live and leaving them to a slow painful death.
Given the choice of being put through a grinder or asphyxiating in CO2 or N2 I would definitely take that gas. The gas is painless and you don't even realise that it's happening.
As I understand it, N2 would have you quietly fall asleep before death, whereas CO2 would have you in pain and terror until your end, since our asphyxiation detection system only accounts for additional CO2, not the lack of oxygen. I expect you meant just CO there?
There is a large difference between killing animals as swiftly as possible to use as much as possible and knicking off a small bit they need to live and leaving them to a slow painful death.