From my perspective, of having been atheist most of my life; the badness was in what I wanted rather than what I did (and in many ways, most people would probably regard me as more good - certainly more likeable and friendly - when I was an atheist than after).
But in retrospect what I wanted was the problem, and I was often trying to overcome my 'natural' and spontaneous ('pagan') goodness so as to be able to be more expedient, experience more pleasure, cease to feel guilty or inhibited about things that I didn't want to do or think - stuff like that.
I was/am since childhood naturally rather puritanical and easily shocked, and this got in the way of fun. So from early adulthood I was pushing against this; trying to persuade myself that life would me more enjoyable if I was more relaxed about things that felt wrong; trying to desensitize myself against that which shocked me.
Also, in a 'cosmic' sense, I tended to assume that there was a purpose to life, that something survived death, that truth was real and important. But my atheist metaphysical assumptions told me that there was no purpose to life and I should forget about everything except This Life - and being truthful was an obstacle to this.
The fact that I just couldn't convince myself that truth was unreal was maybe the factor that broke my atheism - as I observed the whole of science becoming untruthful and corrupting all around me.
In sum; I think that is the worst thing about atheism from the perspective of people 'being good' is that taken seriously it tends to encourage people to get worse, and not to be worried about this.
And if atheism is not taken seriously, not thought-about (as is more common) - it makes people cowardly and compliant to external influences because Why Not? - as we see all around us today.
The de-facto-atheist world (The Global System) now is worse than ever before in recorded history in terms of what it wants; because what the world regards as good and evil are already-and-increasingly inverted; so that we actively seek evil but call it Good.