Abattoir workers
Online child sex abuse investigators
Are 2 in particular that spring to mind. They require people to become numb to pain and suffering.
In anticipation of snarky replies: no, I don't know how else they should be done and yes, I know there's always been pain and suffering. It's at an industrial scale now though.
Maybe it takes a certain kind of human to do it, which is worrying in itself.