man's inhumanity to man
The cruel, barbaric manner in which people treat other people The line appears to have originated in Robert Burns' 1784 poem Man was made to mourn: A Dirge. I'm normally a pretty optimistic person, but reading in the news about man's inhumanity to man day in and day out just makes me want to give up on it all. Just when you think man's inhumanity to man can't get any worse, something horrific like this happens.
man's inhumanity to man
Fig. human cruelty toward other humans. It doesn't take a war to remind us of man's inhumanity to man.