One of the most offensive things a theist can say about us atheists come with death. When people die, it hits much more than it did back when I was still a Christian. As a Christian, I believed the dead person was going to a better place. Without that comfort of heaven, it's horrible.
I will never see them again. They are gone. There is no heaven you'll meet up with them at, where you can say all the things you should of said. You can't apologize for the stupid things you did wrong. You have to suffer with your regrets. There is no redemption.
It makes 'Should have' much more painful. I wish I could believe in heaven and god. It would make death much less horrible.
If you wonder where this all came from: O at Commissionedcomic.com made a comic for his cat who died. If you don't mind crying like crazy like I did, go read it here.
To everyone I've ever wronged: I didn't mean it. I'm sorry. We're all stupid sometimes. I love you all.