If someone is atheist he shouldn't insult others in a bad way just because he is a theist.
But I saw American atheists doing that for Christians. One of them was talking to me saying :
"You don't know what these shitty idiots (Christians ) are doing in America "
In your example the way you arrived at that opinion is by judging the whole by a part, Which is a composition fallacy. Yes there are Atheists that take joy in unnecessarily insulting theists, But there are also many Atheists like myself that recognize that the most productive way to engage a Theist is by having a calm and rational discussion and/or debate.
I have come across a lot of atheists but not all say religion is bad it is just not what they believe. Yes some say religion is bad and say mean things but it has been happening since the beginning of time and that is not going to stop for anything.