How do Atheists explain all the horrible things that mankind is responsible for?

Of course you don't mention any of the good. Your simply a pessimist or blindsided by the media, who reports mainly on violence. Christianity is a good method for spreading peace but you have to suit Spreading The Peace to individual wants and needs. In other words, if Christianity is not for a person, then don't force it, don't use eccentric comments to reinforce your beliefs, and let them find their own way to peace and prosperity. Religion is not for everyone. Some times friends, family, your loved one or/and feeling good about yourself are all that you need to be "saved." Stop focusing on the bad. Of course cultures need to be revised, but form a nation based on one religion, what happens when someone else does not believe in that religion or believes in another religion? They become outcasts. They may be sent to prison or "rehab." Or worse, they're killed. People simply need to be more accepting.