Why does Scout’s question upset Jem in "To Kill a Mockingbird"? Is there a simple answer, or any answer to the question? “How can you hate...

This question puts Jem, essentially, on the same level as Hitler: Both are acting out of some form of hatred. Scout points this out with the simplicity of her question, and by doing so, she redirects Jem's attention from local matters to those more global.

The analogy that the question raises isn't exactly flawless, however: In the case of Hitler, millions of Jews were exterminated out of hate and fear. In the case of Jem, derogatory comments and derivative gossip are the culprits. The two aren't exactly equal, even though they are motivated by the same emotions.