I am an atheist, and i am not against religion. I think that some people can go a little overboard with their religion, but overall would never say that I am against religion. Most of my friends are religious and we get along great. I would however say that I am against people using religion to infringe on people's rights.
By far the biggest destroyer of religion is monotheism, which has extinguished countless faiths throughout Europe, Asia, Africa, Oceania and the Americas -- in fact, on every continent on earth. Monotheists have attacked religion verbally with critique; socially through persecution, slavery and forced conversion; and militarily with genocide.
Some atheists also attack religion -- verbally.
But not all do.
Everyone has their own thoughts but for me, what I think is that Atheist is when you don't believe that there will be a religion and not necessarily means they're against it. But for anti religion, they don't support the religion and are against it. So there is a different meaning for these two words.