I thought the point of Christianity was that everyone deserves mercy?

It is, if you actually believe in it. Unfortunately practically none of them actually do. And discouragingly, most atheists would love for you to believe that the world would be no better off if they actually believed in and followed the teachings of Jesus. It clearly would be. Does that mean Christianity isn’t full of issues? Nope. But it does mean that clearly we’d be better off if people treated each other as they wished to be treated themselves. And yes before the atheists say it, we ALL KNOW this idea is not unique to Christianity. But it’s kind of key to that religion for the small minority of Christians who actually give a shit about it.