It's important to very carefully distinguish exactly what kind of atheism we're talking about here, because there are two main brands, one of which requires a vast amount of 'justification', just like theism. So...
soft atheism- soft atheism means "to lack a positive belief in any god(s)".
hard atheism- means "possessing a positive belief that no god(s) exist".
The difference is very very important to the discussion of "justified belief" mostly because the most common kind of atheism (soft atheism) doesn;t actually make any conjectures or assertions, and therefore requires no actual justification, while the much less common variety of atheism (hard atheism) actually makes a claim about reality (rather than merely rejecting theism, as soft atheism does).
About 95% of atheists are soft atheists, and about 5% of atheists are hard atheists.
Many people understand the difference between these two positions semantically as agnosticism (instead of soft atheism) and atheism (taken to mean hard atheism). Technically speaking, agnosticism is a position regarding knowability, which doesn't necessarily dictate "belief", but that's a subject for another thread.
Theism requires some kind of "justification" in order to be accepted, and so if the "justifications" for theism end up being rejected, then what we are left with is someone who does not posses a belief in god (a soft atheist) which results directly from a lack of justifications toward theism rather than a justification toward hard atheism. In order to be an atheist, you don't need to justify anything whatsoever, you just need to reject the justification of theism.
Regarding hard atheism, which contains a claim about the nature of reality (rather than merely rejecting the justification for a claim like soft-atheism does), justifications rationally would be required, just like the requirement for theistic justification as a requisite for belief. The problem is logistically a proof of hard atheism would be tantamount to scientific evidence that of all possible god(s), none actually exist. This kind of evidence, rest assured, is unfathomably scarce (I find agnosticism to be rational). Whether or not "human suffering" is a kind of justification toward hard atheism makes for a very shitty and unproductive discussion. Not all possible god(s) would conceivably care about human suffering, and therefore human suffering isn't even a necessary or significant contradiction to the existence of "God". And even within the fraction of possible "god(s)" where they conceivably care about human suffering (and therefore are shown to not exist by the existence of human suffering) we still are inevitably making an endless number of an infinite amount of possible assumptions regarding the supposed nature of God, and why or why not they might not be doing anything about human suffering, presuming they care about humans in the first place.
And so now hopefully any reader can easily see why this is a very shitty, less than productive, and irrational discussion to even begin to have in the first place. The only relevance this discussion even has in the real world is to rebuke theists who come up to you, get in your face, and start annoying you with claims that god loves you and everything that happens to you is a part of god's all loving eternal plan.
Do theists really want to spend their time trying to explain why millions of children die every year to fit their warped picture of reality that somehow everything that happens happens because of the Christian gods perfect all loving plan?
I think not... As an agnostic soft-atheist I cannot even enter into this discussion without necessarily entertaining a series of fantastical assumptions that serve no purpose but to degrade and belittle human intellects as a whole.