I'd like to throw in my 2 cents.
I am an Atheist. I am not a member of Reddit, but frequent the website quite often, and am familiar with r/atheism. Everyone is born an atheist. Religion is taught. In the past, anyone who questioned the church was prosecuted, exiled, etc. The non-believers of that time could either speak up or be punished. As times advanced, scientific discoveries and a few brave people that developed arguments against belief in gods, allowed people to challenge religion (with evidence in some cases) and say, for example, "Maybe the earth isn't 6000 years old". This age of questioning led to many of religion's truths being, well, not true. And so with our huge leap in technology and open questioning, people can finally admit publicly they are non-believers with out fear of backlash. Religion has about a 1,950 years of absolute authority over people with basically no major challenge. Just now, non-believers are challenging those teaching and can make their voices heard without fear of backlash.
I do not care what other people believe in, unless:
it effects what is taught in public schools (creationism)
limits peoples' rights (gay marriage, abortion, etc.)
influences scientific progress (stem cells, climate change, etc.)
Since religion influence all of these areas, there will always be a challenge to them. The goal of the challenge being that sound, scientific evidence, or even just common sense and logic is used to make important decisions; not the teachings of a 2000 year old book written by many different men, edited many different times, to fit the ideals of some men.
So while I agree that many of the arguments in r/atheism are in bad taste, this is where many of them are coming from (my opinion). And as people said before, if you don't like it, then don't enter. I don't spend my time on r/christianity or another christian website.
Also, I would like to add, that in many of the comments, there are atheists who are absolutely disgusted with the way r/atheism is handled. So don't judge all atheists by what you see in Reddit just as I don't stereotype all religious people based on the Westboro Baptist Church.
Hope this helps answer your question.