Dr Grandin is autistic. That jarring sense is you being normal. Normal people who feel empathy for something have a hard time killing it.

This plays out in ugly ways in that business -- they either hire psychos who enjoy being cruel _because_ then know that the animals suffer, or the people doing the work convince themselves that the animals are too "dumb" to understand what's happening, and are incapable of suffering. Grandin complains that workers often undo her changes after the fact (e.g. removing a piece of metal that blocks a view of the killing bolt) because they don't understand that the changes aren't to make _their_ jobs easier, and the workers are either constitutionally incapable of understanding how those changes serve the cows, or that the cows deserve any consideration at all.

Any fisherman will tell you that fish can't suffer.