If this sort of topic interests you, I'd highly recommend "Behave" by Robert Sapolsky. A lot of the book examines what is going on in the brains of people who h

If this sort of topic interests you, I'd highly recommend "Behave" by Robert Sapolsky. A lot of the book examines what is going on in the brains of people who have strongly held "us" vs "them" beliefs (i.e. sexists, racists, homophobes). The core thing with these people seems to be essentialist beliefs - believing there is something essentially different and/or wrong with the out group and being too cognitively lazy to view people as individuals or trying to find commonalities. So some people can actually strongly empathize with their in-group and do terrible things to others in the out-group, so I'm not sure I'd say a lack of empathy is 100% the cause of some of humanities worst behavior. More so bad ideas and cognitive laziness. Another thing to mention is that empathy can actually hinder action if you happen to have strong feelings of empathy for someone who is suffering. The reason is because if you're feeling someone else's suffering, you're going to then be focused on how you can stop feeling the suffering yourself instead of how you can help the other person. It has been shown that the stronger you're feeling someone else's suffering, the less capable you will be to help them. Having more compassion is the answer to this problem and if everyone was more compassionate, the world would be immeasurably better.