Here's what you stand for if you're an atheist:
You think you're smarter than some of the greatest minds in history like Rav Joseph B Soloveitchik, the Chafetz Chayim, the Vilna Gaon and countless other great rabbonim whose knowledge, logical abilities and intellect are without peer. Because they believed in God and you thinks that's stupid. That makes you arrogant.
You think you don't have to be grateful for anything you have because no God handed it to you. You don't have to wake up and start your day with Modeh Ani because everything is yours by right. There is no Benefactor in your lives. That makes you selfish.
For those of you who used to be frum and then think you "smartened up", you believe your families and friends are stupid idiots. Well, you'd never come out and say it. You might even want to remain on good terms with them but face it: you think being frum is stupid and that people who believe in God are, chas v'shalom, idiots. Your mothers, your fathers, your siblings, all morons. These people raised you, gave up the best years of their lives to change your diapers, feed you when you couldn't feed yourself, impart eternal and holy values and you essentially spit on it all. What does that say about you?
You think that every Jew who has ever died as a kiddush HaShem, died for nothing because since there's no God they died for nothing. Their beliefs, their passions, their hopes, all wrong according to you. Every Jew, every one of Hitler's victims, every one who disappeared into the Gulag, who burned at the Crusader's stakes, who were tortured and murdered by all the oppressors of our people over the last centuries, you think they died for nothing. Meaningless. They were stupid to have insisted on dying as Jews. What idiots they were, according to you. That, well that makes you something I can't even begin to describe.