Not believing in God is the worst Sin

I was once approached by someone who told me, ‘I am an atheist; I do not believe in God. But I am a very kind and good person. I volunteer in many charitable organizations, and I give to the poor; so, given all that, when I die, will I go to Hell? Is this the justice of you people – just because I don’t believe in God, I should enter Hell?’

An Atheist claim is that, he does all of his good deeds such as volunteering and giving to charity for the good of mankind, not in the hope of reaching Heaven, nor out of fear of going to Hell. So what is he complaining of, then? He should be pleased that he will not be going to heaven, and not be bothered if he is going to hell. After all, he is doing all of his good deeds for the sake of mankind, so he should feel secure in his knowledge of that and it should be sufficient for him.