Thank you for the heads up. It came out wrong actually. I was wondering which is it this threads means in its opinion.Neither.
Atheist terrorism would not be that just because it is done by atheists. Otherwise we could legitimally talk about "dwarf terrorism", "blond terrorism", or whatever.
You would need, at a minimum, some sort of ideological group that condoned violent acts and that had atheism as a core value of its ideology.
And because terrorism needs an ideology of some sort, atheism as a lack of such is simply not enough.
There has probably been, say, nihilist or anarchic terrorism in some scale at some point. And of course, there were violent acts by the Cuban, Soviet and Chinese governments. But it is arguable at the very best whether they qualify as terrorism (as opposed to state actions), let alone as atheistic (as opposed to communist).
I also think you didn't have to say "we", since in bad acts one is judged as an individual, not groups. What that guys called Roof did, does not justify atheism.